Bump typing_extensions dependency

[cdce8p]

Mypy started using typing_extensions.Self which is only available in >=4.0.0.
https://github.com/python/typing_extensions/blob/main/CHANGELOG.md#release-401-november-30-2021

Fixes: #15487

[AlexWaygood]

I checked for other imports using this script:

import ast
import itertools
import pathlib

names = set[str]()

class ImportFinder(ast.NodeVisitor):
    def visit_ImportFrom(self, node: ast.ImportFrom) -> None:
        self.generic_visit(node)
        if node.module != "typing_extensions":
            return
        names.update({obj.name for obj in node.names})

    def visit_Attribute(self, node: ast.Attribute) -> None:
        self.generic_visit(node)
        match node:
            case ast.Attribute(ast.Name("typing_extensions"), attr):
                names.add(attr)
            case _:
                pass


for path in itertools.chain(
    pathlib.Path("mypy").rglob("*.py"),
    pathlib.Path("mypyc").rglob("*.py")
):
    ImportFinder().visit(ast.parse(path.read_text()))

print(names)

The is_typeddict one was the only other newish import from typing_extensions that the script surfaced.

[AlexWaygood]

We should probably also update mypy-requirements.txt

[AlexWaygood]

LGTM, thanks!

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Thanks all!