Rework Match.group handling

[srittau]

Standardize group(), groups(), groupdict(), and __getattr__() to return AnyStr | Any instead of AnyStr. Also special case group(0) and __getattr__(0) to always return AnyStr. Correctly handle defaults. Use PEP 604 for unions in changed lines.

Edited to include changes after review below.

Fixes #3902

[Akuli]

I'm arfaid this will cause dead code warnings in mypy, if you do something like this:

foo = match.group(1)
if foo is None:
    ...
else:
    ...

Having type checkers say "this is a string" has caused confusion before. See #5269 (comment) for example.

[srittau]

Maybe we should just use AnyStr | Any here as well. I will try that.

[Akuli]

Seems like it would also be better for mypyc (it's just one tool that uses typeshed, but an important tool): #5269 (comment)

[github-actions]

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

sphinx (https://github.com/sphinx-doc/sphinx.git)
+ sphinx/ext/autodoc/__init__.py: note: In member "parse_name" of class "Documenter":
+ sphinx/ext/autodoc/__init__.py:405:23: error: Need type annotation for "parents"

pydantic (https://github.com/samuelcolvin/pydantic.git)
+ pydantic/networks.py:267: error: Item "None" of "Optional[Match[str]]" has no attribute "group"  [union-attr]

werkzeug (https://github.com/pallets/werkzeug.git)
+ src/werkzeug/http.py:456: error: Unsupported operand types for + ("str" and "None")
+ src/werkzeug/http.py:456: note: Right operand is of type "Optional[str]"
+ src/werkzeug/http.py:458: error: Incompatible types in assignment (expression has type "Optional[str]", target has type "str")

aiohttp (https://github.com/aio-libs/aiohttp.git)
+ aiohttp/client_reqrep.py:809: error: unused "type: ignore" comment
+ aiohttp/client_reqrep.py:813: error: Argument 1 to "add" of "MultiDict" has incompatible type "Union[str, Any, URL]"; expected "Union[str, istr]"  [arg-type]

[srittau]

The primer output looks tolerable in my opinion, but we should wait to merge this to after the next mypy release, though, to minimize disruption.

[srittau]

Now that mypy has released, I'd consider this ready for merge.