fix(gapic-generator): remove Python 3.9 from unit tests

This removes the temporary skip for Python 3.9 in the unit test session and removes it from ALL_PYTHON, completing the drop of Python 3.9 support.

Author: chalmerlowe

packages/gapic-generator/noxfile.py

@@ -44,7 +44,6 @@
 RUFF_EXCLUDES = "*golden*,*pb2.py,*pb2.pyi"
 
 ALL_PYTHON = (
-    "3.9",
     "3.10",
     "3.11",
     "3.12",
@@ -58,8 +57,7 @@
 @nox.session(python=ALL_PYTHON)
 def unit(session):
     """Run the unit test suite."""
-    if session.python == "3.9":
-        session.skip("Skipping Python 3.9 unit tests temporarily.")
+
     session.install(
         # TODO(https://github.com/googleapis/gapic-generator-python/issues/2478):
         # Temporarily pin coverage to 7.11.0