fix(sqlalchemy-spanner): Drop support for Python 3.8 and 3.9 (#16913)

This PR updates `sqlalchemy-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/sqlalchemy-spanner/noxfile.py

@@ -80,11 +80,10 @@ class = StreamHandler
 BLACK_VERSION = "black==23.7.0"
 ISORT_VERSION = "isort==5.11.0"
 BLACK_PATHS = ["google", "tests", "noxfile.py", "setup.py", "samples"]
-UNIT_TEST_PYTHON_VERSIONS = ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+UNIT_TEST_PYTHON_VERSIONS = ["3.10", "3.11", "3.12", "3.13", "3.14"]
 ALL_PYTHON = list(UNIT_TEST_PYTHON_VERSIONS)
-ALL_PYTHON.extend(["3.7"])
 SYSTEM_TEST_PYTHON_VERSIONS = ["3.12"]
-SYSTEM_COMPLIANCE_MIGRATION_TEST_PYTHON_VERSIONS = ["3.8", "3.12", "3.14"]
+SYSTEM_COMPLIANCE_MIGRATION_TEST_PYTHON_VERSIONS = ["3.12", "3.14"]
 DEFAULT_PYTHON_VERSION = "3.14"
 DEFAULT_PYTHON_VERSION_FOR_SQLALCHEMY_20 = "3.14"
 
@@ -180,6 +179,8 @@ def compliance_test_14(session):
         "--asyncio-mode=auto",
         "tests/test_suite_14.py",
         *session.posargs,
+        # Silence SQLAlchemy 2.0 transition warnings for this 1.4 compatibility session.
+        env={"SQLALCHEMY_SILENCE_UBER_WARNING": "1"},
     )
 
 
@@ -318,8 +319,6 @@ def _migration_test(session):
 @nox.parametrize("test_type", ["unit", "mockserver"])
 def unit(session, test_type):
     """Run unit tests."""
-    if session.python in ("3.7",):
-        session.skip("Python 3.7 is no longer supported")
 
     if (
         test_type == "mockserver"

packages/sqlalchemy-spanner/setup.py

@@ -58,11 +58,10 @@
     author="Google LLC",
     author_email="googleapis-packages@google.com",
     license="Apache 2.0",
+    python_requires=">=3.10",
     classifiers=[
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",
-        "Programming Language :: Python :: 3.8",
-        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
         "Programming Language :: Python :: 3.12",