This makes `Git.AutoInterrupt` and `Git.CatFileContentStream`
transparent aliases to top-level nonpublic `_AutoInterrupt` and
`_CatFileContentStream` classes in the `cmd` module.

This does not change the "public" interface. It also does not
change metadata relevant to documentation: the `__name__` and
`__qualname__` attributes are set explicitly to the values they had
before when these classes were defined nested, so that Sphinx
continues to document them (and to do so in full) in `Git` and as
`Git.AutoInterrupt` and `Git.CatFileContentStream`.

The purpose of this is to increase readability. The `Git` class is
big and complex, with a number of long members and various forms of
nesting. Since these two classes can be understood even without
reading the code of the `Git` class, moving the definitions out of
the `Git` class into top-level nonpublic classes will hopefully
increase readability and help with maintenance.

EliahKagan added a commit to EliahKagan/GitPython that referenced this pull request

This uses `TypeAlias` from the `typing` module, to make it so the
assignment statments introduced in gitpython-developers#2037 (to set `Git.AutoInterrupt`
and `Git.CatFileContentStream` to nonpublic module-level
implementations `_AutoInterrupt` and `_CatFileContentStream`) are
treated by `mypy` as type aliases rather than as class variables.

For details on the problem this partially fixes, see gitpython-developers#2038 and:
https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases

The fix won't work in this form, however, because it attempts to
import `TypeAlias` unconditionally from the standard-library
`typing` module, which only gained it in Python 3.10.

This was referenced