Update configparser.SectionProxy.get* to return T | None when fallback is not specified

[brianschubert]

Refs #12933

Exploratory PR to see what the mypy_primer impact is.

[github-actions]

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

porcupine (https://github.com/Akuli/porcupine)
+ porcupine/plugins/editorconfig.py:98: error: Item "None" of "str | None" has no attribute "lower"  [union-attr]

nox (https://github.com/wntrblm/nox)
+ nox/tox_to_nox.py:89: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
+ nox/tox_to_nox.py:121: error: Argument 1 to "Path" has incompatible type "Optional[str]"; expected "Union[str, PathLike[str]]"  [arg-type]

[brianschubert]

mypy_primer hits look good:

• porcupine: uses .get("root", "false"). This will go away with #12935.
• nox: first hit uses .get("set_env", ""), which will go away with #12935. Second hit uses .get("change_dir"), which is a true hit.