Remove type aliases that are long supported

[sobolevn]

Some builtin aliases are available for all python versions that we support. So, there's no need to check them in semanal:

mypy/mypy/semanal.py

Lines 673 to 689 in 8738886

	def add_builtin_aliases(self, tree: MypyFile) -> None:
	"""Add builtin type aliases to typing module.
	For historical reasons, the aliases like `List = list` are not defined
	in typeshed stubs for typing module. Instead we need to manually add the
	corresponding nodes on the fly. We explicitly mark these aliases as normalized,
	so that a user can write `typing.List[int]`.
	"""
	assert tree.fullname == "typing"
	for alias, target_name in type_aliases.items():
	if type_aliases_source_versions[alias] > self.options.python_version:
	# This alias is not available on this Python version.
	continue
	name = alias.split(".")[-1]
	if name in tree.names and not isinstance(tree.names[name].node, PlaceholderNode):
	continue
	self.create_alias(tree, target_name, alias, name)

[github-actions]

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

vision (https://github.com/pytorch/vision): typechecking got 1.28x slower (44.0s -> 56.3s)
(Performance measurements are based on a single noisy sample)

[ikonst]

Eh, wish we had magic trailing comma on.

[sobolevn]

It is a @hauntsaninja's preference :)
I prefer , everywhere.