Start on fixing AutoInterrupt/CatFileContentStream aliases

This uses `TypeAlias` from the `typing` module, to make it so the
assignment statments introduced in #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 #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.

git/cmd.py

@@ -55,6 +55,7 @@
     TYPE_CHECKING,
     TextIO,
     Tuple,
+    TypeAlias,
     Union,
     cast,
     overload,
@@ -952,9 +953,9 @@ def check_unsafe_options(cls, options: List[str], unsafe_options: List[str]) ->
                         f"{unsafe_option} is not allowed, use `allow_unsafe_options=True` to allow it."
                     )
 
-    AutoInterrupt = _AutoInterrupt
+    AutoInterrupt: TypeAlias = _AutoInterrupt
 
-    CatFileContentStream = _CatFileContentStream
+    CatFileContentStream: TypeAlias = _CatFileContentStream
 
     def __init__(self, working_dir: Union[None, PathLike] = None) -> None:
         """Initialize this instance with: