Import TypeAlias from typing_extensions where needed

The standard library `typing` module introduced `TypeAlias` in
Python 3.10. This uses it from `typing_extensions` where neederd,
by making three changes:

- Change the version lower bound for `typing-extensions` from
  3.7.4.3 to 3.10.0.2, since 3.7.4.3 doesn't offer `TypeAlias`.

  (The reason not to go higher, to major version 4, is that
  it no longer supports versions of Python lower than 3.9, but
  we currently support Python 3.7 and Python 3.8.)

- Require the `typing-extensions` dependency when using Python
  versions lower than 3.10, rather than only lower than 3.7 as
  before.

- Conditionally import `TypeAlias` (in the `git.cmd` module) from
  either `typing` or `type_extensions` depending on the Python
  version, using a pattern that `mypy` and other type checkers
  recognize statically.

Together with the preceding commit, this fixes #2038. (This is
approach (2) described there.)

Author: EliahKagan

git/cmd.py

@@ -55,12 +55,16 @@
     TYPE_CHECKING,
     TextIO,
     Tuple,
-    TypeAlias,
     Union,
     cast,
     overload,
 )
 
+if sys.version_info >= (3, 10):
+    from typing import TypeAlias
+else:
+    from typing_extensions import TypeAlias
+
 from git.types import Literal, PathLike, TBD
 
 if TYPE_CHECKING:

requirements.txt

@@ -1,2 +1,2 @@
 gitdb>=4.0.1,<5
-typing-extensions>=3.7.4.3;python_version<"3.8"
+typing-extensions>=3.10.0.2;python_version<"3.10"