Mention collections.abc for Sequence

In the deprecation comment for importing Sequence from the top
level git module. Although the git module imports it from typing,
collections.abc.Sequence supports __class_getitem__ since Python
3.9 (and typing.Sequence is actually itself deprecated since then).

Author: EliahKagan

git/__init__.py

@@ -58,7 +58,7 @@
     "RepositoryDirtyError",
     "RootModule",
     "RootUpdateProgress",
-    "Sequence",  # Deprecated - import this from `typing` instead.
+    "Sequence",  # Deprecated - import from `typing`, or `collections.abc` in Python 3.9+.
     "StageType",
     "Stats",
     "Submodule",