RemoteProgress._parse_progress_line may merit a broader return type

[EliahKagan]

RemoteProgress._parse_progress_line is conceptually a "protected" method: code outside GitPython may subclass RemoteProgress and override it with another implementation (which may also reasonably call the base class implementation).

The method is conceptually void as implemented in the base class, always implicitly returning None, and it is annotated accordingly:

https://github.com/gitpython-developers/GitPython/blob/afa575454f85b34800460881cc9c3cd7fe78c8e3/git/util.py#L582

However, while it would not ordinarily be useful to do so, it is--or has been--also intended that it be overridable to return a non-None value. This is mostly for compatibility with existing code that may have done so in the past, but it may be that reasonable cases for doing so in new code can still arise. This is reflected in code of GitPython:

https://github.com/gitpython-developers/GitPython/blob/afa575454f85b34800460881cc9c3cd7fe78c8e3/git/util.py#L685-L697

And in the subclass of RemoteProgress introduced in the test suite, which overrides the method:

https://github.com/gitpython-developers/GitPython/blob/afa575454f85b34800460881cc9c3cd7fe78c8e3/test/test_remote.py#L54-L59

But any override that returns a non-None value is currently inconsistent with the return type annotation on the base class, which is written as -> None. For Liskov substitutability, an overridden method's return type must be the same or a subclass of the base method's return type, and if a subclass has type annotations, and the return type is specified as a non-Any type, type checkers such as mypy and pyright will report a non-None return type as a violation.

If possible, the return type in RemoteProgress should be broadened, but in a way that does not unduly encourage subclasses to return non-None. See comments on #1788, starting at https://github.com/gitpython-developers/GitPython/pull/1788#issuecomment-1869653326, for a discussion and analysis of this. This issue, and the above recommendation, is based on the findings and discussion there.