fix(spanner): drop support for Python 3.9

Python 3.9 is EOL and missing from the Kokoro test image, causing nightly failures.

- Remove "3.9" from noxfile.py ALL_PYTHON and sessions
- Remove "3.9" from setup.py classifiers and update python_requires to >=3.10

BREAKING CHANGE: Python 3.9 is no longer supported. Minimum supported version is now 3.10.

Author: sakthivelmanii

packages/google-cloud-spanner/noxfile.py

@@ -31,7 +31,6 @@
 SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.12"]
 
 ALL_PYTHON: List[str] = [
-    "3.9",
     "3.10",
     "3.11",
     "3.12",
@@ -71,7 +70,6 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",

packages/google-cloud-spanner/setup.py

@@ -86,7 +86,6 @@
         "License :: OSI Approved :: Apache Software License",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
         "Programming Language :: Python :: 3.12",
@@ -97,7 +96,7 @@
     ],
     platforms="Posix; MacOS X; Windows",
     packages=packages,
-    python_requires=">=3.9",
+    python_requires=">=3.10",
     install_requires=dependencies,
     extras_require=extras,
     include_package_data=True,