fix(testutils): Drop support for Python 3.9 (#16948)

This PR updates \`google-cloud-testutils\` 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 3.9.
* Constraints: Transferred lower bounds to \`constraints-3.10.txt\` and
dropped \`constraints-3.9.txt\`.
* Cleanup: Removed obsolete \`tests/test_version_warnings.py\`.
* Expert Improvements: Modernized \`mypy.ini\` to 3.10 and added \`#
version-scanner: ignore\` pragmas to test fixtures.

Verified successfully with 35 passing unit tests under Python 3.10!

Fixes internal issue: http://b/482126936 🦕

Author: chalmerlowe

packages/google-cloud-testutils/mypy.ini

@@ -1,3 +1,3 @@
 [mypy]
-python_version = 3.9
+python_version = 3.10
 exclude = tests/unit/resources/

packages/google-cloud-testutils/noxfile.py

@@ -33,7 +33,7 @@
 # Error if a python version is missing
 nox.options.error_on_missing_interpreters = True
 
-ALL_PYTHON = ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+ALL_PYTHON = ["3.10", "3.11", "3.12", "3.13", "3.14"]
 DEFAULT_PYTHON_VERSION = "3.14"
 BLACK_VERSION = "black==23.7.0"
 RUFF_VERSION = "ruff==0.14.14"
@@ -157,7 +157,7 @@ def system(session):
         "--package-name",
         "google-cloud-testutils",
         "--constraints-file",
-        "testing/constraints-3.9.txt",
+        "testing/constraints-3.10.txt",
     )
 
 
@@ -171,7 +171,7 @@ def update_lower_bounds(session):
         "--package-name",
         "google-cloud-testutils",
         "--constraints-file",
-        "testing/constraints-3.9.txt",
+        "testing/constraints-3.10.txt",
     )
 
 

packages/google-cloud-testutils/setup.py

@@ -55,17 +55,16 @@
     platforms="Posix; MacOS X; Windows",
     include_package_data=True,
     install_requires=(
-        "google-auth >= 0.4.0",
+        "google-auth >= 2.1.0",
         "click>=7.0.0",
         "packaging>=22.0",
     ),
-    python_requires=">=3.9",
+    python_requires=">=3.10",
     classifiers=[
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",
         "Operating System :: OS Independent",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
         "Programming Language :: Python :: 3.12",

packages/google-cloud-testutils/testing/constraints-3.10.txt

@@ -0,0 +1,10 @@
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+click==7.0.0
+google-auth==2.1.0
+packaging==22.0

packages/google-cloud-testutils/testing/constraints-3.9.txt

@@ -36,5 +36,5 @@
     ],
     install_requires=requirements,
     packages=setuptools.find_packages(),
-    python_requires=">=3.9",
+    python_requires=">=3.9",  # version-scanner: ignore
 )

packages/google-cloud-testutils/tests/unit/resources/bad_package/setup.py

@@ -42,5 +42,5 @@
     install_requires=requirements,
     extras_require=extras,
     packages=setuptools.find_packages(),
-    python_requires=">=3.9",
+    python_requires=">=3.9",  # version-scanner: ignore
 )