Micro-optimize flatten_nested_unions

[JukkaL]

Avoid constructing a new list if there is nothing to flatten (and the input is a list, which if often the case).

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

I think we only need to convert it to a list if we're taking the fast path, correct?

Suggested change

	if not isinstance(types, list):
	typelist = list(types)
	else:
	typelist = cast("list[Type]", types)
	# Fast path: most of the time there is nothing to flatten
	if not any(isinstance(t, (TypeAliasType, UnionType)) for t in typelist): # type: ignore[misc]
	return typelist
	# Fast path: most of the time there is nothing to flatten
	if not any(isinstance(t, (TypeAliasType, UnionType)) for t in types): # type: ignore[misc]
	if not isinstance(types, list):
	return list(types)
	return cast("list[Type]", types)

[JukkaL]

I'm first converting it to a list since iterating over a list if faster than iterating over a sequence, and since we need to return a list anyway, so it would only delay the construction of the list. Since we take the fast path the vast majority of the time, the redundant construction of a list in the slow path is not a big deal, and we save some of the cost through faster iteration. If mypyc was smarter about iterating over sequences, your suggested approach would be better.

[AlexWaygood]

Thanks for the explanation!