Fix @takes_callable_and_args TypeVar binding bug

[gschaffner]

resolves #71.

the change in the plugin's behavior bisects to python/mypy#14095, which changes the signature generated by the plugin:

• before:

  reveal_type(nursery.start_soon)
  # note: Revealed type is "Overload(
  #     ...,  # other overloads
  #     def [__T1] (async_fn: def [__T1] (__T1`17) -> typing.Awaitable[Any], __T1`17, *, name: builtins.object =),
  #     ...,  # other overloads
  # )"

• after:

  reveal_type(nursery.start_soon)
  # note: Revealed type is "Overload(
  #     ...,  # other overloads
  #     def [__T1] (async_fn: def [__T1] (__T1`93) -> typing.Awaitable[Any], __T1`94, *, name: builtins.object =),
  #     ...,  # other overloads
  # )"

  that is, what used to be one __T1 binding is now two different ones!

cc @oremanj, as this is your code :)

if merged, a new release should probably be made as this is effecting people in the wild.

[belm0]

Will this fix, in turn, cause errors from older mypy?

[oremanj]

I tested the new code against an old mypy (0.982) and it produced the same correct-looking reveal_type output. It also seems to type-check correctly. Going to land it and hope for the best.

[gschaffner]

sorry for the delay!

but yeah, this should be fine on older mypy versions. in particular ci.sh tested this on the oldest mypy version supported by trio-typing.