fix: retain the typing Union compatibility import

Demisto SDK imports `Union` from `git` while registering validators. Released
GitPython exposes `typing.Union`; removing that incidental export prevents
its unchanged root test conftest and CLI command registration from importing.

Retain the explicit alias without changing wildcard exports or adding Git
behavior. This unblocks the existing caller while preserving the narrower
package namespace elsewhere.

The import regression failed before this change. All 35 downstream
compatibility cases pass on CLI and Gix. Ruff lint/format, mypy for
`git/__init__.py`, and `git diff --check` pass. Released GitPython 3.2.1
was force-installed and verified in site-packages for the downstream baseline.

Author: codex

doc/source/changes.rst

@@ -51,6 +51,8 @@ Git executable and safety
 API changes
 -----------
 
+* Explicit ``from git import Union`` imports remain available for Demisto SDK's
+  validator registration, retaining the historical ``typing.Union`` alias.
 * Editable/source imports expose the release version in ``git.__version__``,
   matching built distributions. Salt's GitPython provider checks can parse the
   version instead of rejecting the unstamped ``"git"`` build placeholder.

git/__init__.py

@@ -88,6 +88,7 @@
     __version__ = (_Path(__file__).resolve().parents[1] / "VERSION").read_text(encoding="utf-8").strip()
 
 from typing import Optional as _Optional
+from typing import Union as Union  # Compatibility for downstream explicit imports.
 
 from git.util import to_hex_sha
 

test/test_downstream_compatibility.py

@@ -1,6 +1,7 @@
 """Public GitPython queries used by released downstream projects."""
 
 from pathlib import Path
+from typing import Union
 from importlib.metadata import version
 
 import pytest
@@ -15,6 +16,12 @@ def test_public_version_supports_downstream_provider_checks():
     assert git.__version__ == version("GitPython")
 
 
+def test_legacy_typing_union_import_supports_downstream_registration():
+    from git import Union as GitUnion
+
+    assert GitUnion is Union
+
+
 @pytest.mark.parametrize("object_format", ["sha1", "sha256"])
 @pytest.mark.parametrize("ref_format", ["files", "reftable"])
 def test_repo_accepts_explicit_literal_path_selection(object_format, ref_format, monkeypatch):