Fix new style union syntax in type aliases

[JukkaL]

Fix Python 3.10 | union syntax in type aliases, when one of
the operands is a type alias or a type with an overloaded __init__.

Fixes #12368. Fixes #12005. Fixes #11426.

[JukkaL]

Some tests were using the typeshed stub for functools, and it requires typing._SpecialForm. Instead of adding a bunch of typing-medium.pyi fixtures, I decided to create a simplified stub to use in tests. This also speeds up the test cases a bit.

[hauntsaninja]

pandas looks like a real issue, they probably meant to do _truthy_value: Scalar instead of _truthy_value = Scalar in the base class

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/annotations.py:1064: error: Dict entry 6 has incompatible type "object": "Tuple[ToUser, ToRole]"; expected "Type[Any]": "Tuple[Union[Callable[[str], Coroutine[Any, Any, Any]], Callable[[str], Any]], ...]"  [dict-item]
+ tanjun/annotations.py:1064: error: Dict entry 6 has incompatible type "<typing special form>": "Tuple[ToUser, ToRole]"; expected "Type[Any]": "Tuple[Union[Callable[[str], Coroutine[Any, Any, Any]], Callable[[str], Any]], ...]"  [dict-item]
- tanjun/annotations.py:1072: error: List item 4 has incompatible type "object"; expected "Type[Any]"  [list-item]
+ tanjun/annotations.py:1072: error: List item 4 has incompatible type "<typing special form>"; expected "Type[Any]"  [list-item]
- tanjun/annotations.py:1247: error: Dict entry 6 has incompatible type "object": "Callable[[_ArgConfig, SlashCommand[Any], str], Self?]"; expected "Type[Any]": "Callable[[_ArgConfig, SlashCommand[Any], str], SlashCommand[Any]]"  [dict-item]
+ tanjun/annotations.py:1247: error: Dict entry 6 has incompatible type "<typing special form>": "Callable[[_ArgConfig, SlashCommand[Any], str], Self?]"; expected "Type[Any]": "Callable[[_ArgConfig, SlashCommand[Any], str], SlashCommand[Any]]"  [dict-item]

discord.py (https://github.com/Rapptz/discord.py)
- discord/app_commands/transformers.py:728: error: Dict entry 0 has incompatible type "AppCommandOptionType": "Tuple[Type[str], object]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
+ discord/app_commands/transformers.py:728: error: Dict entry 0 has incompatible type "AppCommandOptionType": "Tuple[Type[str], <typing special form>]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
- discord/app_commands/transformers.py:729: error: Dict entry 1 has incompatible type "AppCommandOptionType": "Tuple[Type[int], object]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
+ discord/app_commands/transformers.py:729: error: Dict entry 1 has incompatible type "AppCommandOptionType": "Tuple[Type[int], <typing special form>]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
- discord/app_commands/transformers.py:730: error: Dict entry 2 has incompatible type "AppCommandOptionType": "Tuple[Type[bool], object]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
+ discord/app_commands/transformers.py:730: error: Dict entry 2 has incompatible type "AppCommandOptionType": "Tuple[Type[bool], <typing special form>]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
- discord/app_commands/transformers.py:731: error: Dict entry 3 has incompatible type "AppCommandOptionType": "Tuple[Type[float], object]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
+ discord/app_commands/transformers.py:731: error: Dict entry 3 has incompatible type "AppCommandOptionType": "Tuple[Type[float], <typing special form>]"; expected "AppCommandOptionType": "Tuple[Type[Any], ...]"  [dict-item]
- discord/app_commands/transformers.py:738: error: Incompatible default for argument "_none" (default has type "object", argument has type "type")  [assignment]
+ discord/app_commands/transformers.py:738: error: Incompatible default for argument "_none" (default has type "<typing special form>", argument has type "type")  [assignment]
- discord/app_commands/commands.py:254: error: Incompatible default for argument "_none" (default has type "object", argument has type "type")  [assignment]
+ discord/app_commands/commands.py:254: error: Incompatible default for argument "_none" (default has type "<typing special form>", argument has type "type")  [assignment]
- discord/ext/commands/converter.py:1177: error: Argument 2 to "issubclass" has incompatible type "_SpecialForm"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"  [arg-type]
+ discord/ext/commands/converter.py:1177: error: Argument 2 to "issubclass" has incompatible type "<typing special form>"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"  [arg-type]

pandera (https://github.com/pandera-dev/pandera)
- pandera/dtypes.py:23: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "typing._SpecialForm")  [assignment]
+ pandera/dtypes.py:23: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "<typing special form>")  [assignment]
- pandera/schemas.py:46: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "typing._SpecialForm")  [assignment]
+ pandera/schemas.py:46: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "<typing special form>")  [assignment]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/integer.py:135: error: Incompatible types in assignment (expression has type "int", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]
+ pandas/core/arrays/integer.py:136: error: Incompatible types in assignment (expression has type "int", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]
+ pandas/core/arrays/floating.py:119: error: Incompatible types in assignment (expression has type "float", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]
+ pandas/core/arrays/floating.py:120: error: Incompatible types in assignment (expression has type "float", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]
+ pandas/core/arrays/boolean.py:290: error: Incompatible types in assignment (expression has type "bool", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]
+ pandas/core/arrays/boolean.py:291: error: Incompatible types in assignment (expression has type "bool", base class "BaseMaskedArray" defined the type as "<typing special form>")  [assignment]