chore(django-google-spanner): update Python version to 3.10 (#16914)

This PR updates `django-google-spanner` to establish Python 3.10 as the
minimum supported version, dropping support for Python 3.9 and below.

### Changes

* Configuration: Updated `setup.py` and `noxfile.py` to require Python
3.10+ and remove references to Python 3.7, 3.8, and 3.9.
* Cleanup: Removed dead code in `noxfile.py` that was skipping Python
3.7.

### Blocking

This PR is blocked by PR #16910

Author: chalmerlowe

packages/django-google-spanner/noxfile.py

@@ -28,7 +28,6 @@
 DEFAULT_PYTHON_VERSION = "3.14"
 
 UNIT_TEST_PYTHON_VERSIONS = [
-    "3.9",
     "3.10",
     "3.11",
     "3.12",
@@ -123,8 +122,7 @@ def default(session, django_version="5.2"):
 @nox.session(python=ALL_PYTHON)
 def unit(session):
     """Run the unit test suite."""
-    if session.python == "3.9":
-        session.skip("Python 3.9 is not supported for Django 5.2 tests")
+    # TODO: Remove this check once support for Python 3.14 is added to Protobuf
     if session.python == "3.14":
         session.skip("Protobuf upb implementation is not supported in Python 3.14 yet")
     print("Unit tests with django 5.2")