feat: Drop support for Python 3.7 and 3.8

Removes support for Python 3.7 and 3.8, establishing Python 3.9 as the new minimum supported version.

This change involves:
- Updating `python_requires` and classifiers in `setup.py`.
- Modifying Python versions in `noxfile.py` (default, unit tests, system tests) and ensuring constraint file logic remains correct.
- Updating the GitHub Actions workflow (`unittest.yml`) matrix, runner, and coverage job version.
- Deleting constraint files for Python 3.7 and 3.8 (`testing/constraints-3.7.txt`, `testing/constraints-3.8.txt`).
- Removing Kokoro sample configuration directories (`.kokoro/samples/python3.7/`, `.kokoro/samples/python3.8/`).
- Updating supported version mentions in `README.rst`.
- Removing 3.7 and 3.8 from the `ALL_VERSIONS` list in `samples/snippets/noxfile.py`.

Author: google-labs-jules[bot]

.github/workflows/unittest.yml

@@ -5,13 +5,11 @@ on:
 name: unittest
 jobs:
   unit:
-    # TODO(https://github.com/googleapis/gapic-generator-python/issues/2303): use `ubuntu-latest` once this bug is fixed.
-    # Use ubuntu-22.04 until Python 3.7 is removed from the test matrix
-    # https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
-    runs-on: ubuntu-22.04
+    # Use `ubuntu-latest` runner.
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: ['3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
+        python: ['3.9', '3.10', '3.11', '3.12', '3.13']
     steps:
     - name: Checkout
       uses: actions/checkout@v4
@@ -103,7 +101,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v5
       with:
-        python-version: "3.8"
+        python-version: "3.9"
     - name: Install coverage
       run: |
         python -m pip install --upgrade setuptools pip wheel

.kokoro/samples/python3.7/common.cfg

@@ -34,11 +34,11 @@ dependencies.
 
 Supported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^
-Python >= 3.7
+Python >= 3.9
 
 Unsupported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Python <= 3.6.
+Python <= 3.8.
 
 
 Mac/Linux

.kokoro/samples/python3.7/continuous.cfg

@@ -32,11 +32,9 @@
 ISORT_VERSION = "isort==5.11.0"
 LINT_PATHS = ["docs", "db_dtypes", "tests", "noxfile.py", "setup.py"]
 
-DEFAULT_PYTHON_VERSION = "3.8"
+DEFAULT_PYTHON_VERSION = "3.9"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
-    "3.7",
-    "3.8",
     "3.9",
     "3.10",
     "3.11",
@@ -56,7 +54,7 @@
 UNIT_TEST_EXTRAS: List[str] = []
 UNIT_TEST_EXTRAS_BY_PYTHON: Dict[str, List[str]] = {}
 
-SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.8"]
+SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.9"]
 SYSTEM_TEST_STANDARD_DEPENDENCIES: List[str] = [
     "mock",
     "pytest",

.kokoro/samples/python3.7/periodic-head.cfg

@@ -89,7 +89,7 @@ def get_pytest_env_vars() -> Dict[str, str]:
 
 # DO NOT EDIT - automatically generated.
 # All versions used to test samples.
-ALL_VERSIONS = ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+ALL_VERSIONS = ["3.9", "3.10", "3.11", "3.12", "3.13"]
 
 # Any default versions that should be ignored.
 IGNORED_VERSIONS = TEST_CONFIG["ignored_versions"]

.kokoro/samples/python3.7/periodic.cfg

@@ -63,8 +63,6 @@ def readme():
         "License :: OSI Approved :: Apache Software License",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.7",
-        "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
@@ -75,6 +73,6 @@ def readme():
     ],
     platforms="Posix; MacOS X; Windows",
     install_requires=dependencies,
-    python_requires=">=3.7",
+    python_requires=">=3.9",
     tests_require=["pytest"],
 )