configparser: allow default_section=None

[ekanshul]

Closes #12700

Widen RawConfigParser.default_section and the default_section parameter of RawConfigParser.__init__ to str | None, since passing None works at runtime to disable the default section and is relied on in the wild (e.g. setuptools).

Agent used: Claude Code

[github-actions]

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

setuptools (https://github.com/pypa/setuptools)
+ setuptools/config/expand.py:371: error: Unused "type: ignore" comment  [unused-ignore]
+ setuptools/config/expand.py:372: error: Cannot assign to a method  [method-assign]
+ setuptools/config/expand.py:372: error: Incompatible types in assignment (expression has type "type[str]", variable has type "Callable[[str], str]")  [assignment]
+ setuptools/config/expand.py:375: error: Argument 1 to "pop" of "dict" has incompatible type "str | None"; expected "str"  [arg-type]