chore(versions): update Python version support to 3.10+ in gapic-generator (#16910)

> [!note]
> This PR has been updated to remove 3.7, 3.8, and 3.9 (with one
exception discussed below).

### Overview
This Pull Request updates the gapic-generator package and related CI
configurations to reflect the deprecation of Python 3.7 through 3.9. It
updates documentation, templates, and CI scripts to establish Python
3.10 as the new minimum supported version where applicable.

### Changes
#### Documentation:
* Updated installation and verification instructions in
packages/gapic-generator/docs to reference Python 3.10
#### Templates:
* Updated __init__.py.j2 templates in gapic/ads-templates to require
Python 3.10+ (updating sys.version_info checks and error messages).
* Removed outdated comments regarding Python 3.9 and below features
(like module-level __getattr__) to avoid confusion, as the baseline
runtime is now higher than the version noted in the comments.
* Updated mypy.ini.j2 to target Python 3.10.
* Cleaned up dead code (e.g. commented-out Jinja snippets) in the
templates.
#### CI Configurations:
* Removed Python 3.9 and below from the unit test matrix in
ci/run_single_test.sh.
* Updated comments regarding supported Python versions in
ci/run_conditional_tests.sh.

### Reasoning
Python 3.9 and below have reached End-of-Life (EOL) and are no longer
supported. These changes ensure that our generator and the libraries it
produces align with current support policies and reduce maintenance
burden by removing dead code paths.

> [!note]
> because the config that runs this PR checks is not updated yet, it
still wants to run 3.9 so we have a temp change to skip that nox
session. Once this PR merges future PRs will no longer attempt to run
3.9 and we will follow-up with a small PR to strip out that last 3.9
reference.



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

Author: chalmerlowe

.generator/Dockerfile

@@ -66,6 +66,7 @@ RUN unzip protoc-25.3-linux-x86_64.zip -d protoc
 
 # Download/extract pandoc
 # Pandoc is required by gapic-generator-python for parsing documentation
+# version-scanner: ignore-next-line
 ENV PANDOC_VERSION=3.8.2
 RUN mkdir pandoc-binary
 RUN wget https://github.com/jgm/pandoc/releases/download/${PANDOC_VERSION}/pandoc-${PANDOC_VERSION}-linux-amd64.tar.gz

.generator/cli.py

@@ -354,7 +354,7 @@ def _run_post_processor(output: str, library_id: str, is_mono_repo: bool):
         # TODO(https://github.com/googleapis/google-cloud-python/issues/15538):
         # Investigate if a `target_version needs to be maintained
         # or can be eliminated.
-        target_version = "py39"
+        target_version = "py310"
         common_args = [
             f"--target-version={target_version}",
             "--line-length=88",

.generator/test_cli.py

@@ -741,7 +741,7 @@ def test_run_individual_session_success(mocker, caplog, is_mono_repo):
         "cli.subprocess.run", return_value=MagicMock(returncode=0)
     )
 
-    test_session = "unit-3.9"
+    test_session = "unit-3.10"
     test_library_id = "test-library"
     repo = "repo"
     _run_individual_session(test_session, test_library_id, repo, is_mono_repo)

.github/workflows/gapic-generator-tests.yml

@@ -18,7 +18,7 @@ env:
   SHOWCASE_VERSION: 0.35.0
   PROTOC_VERSION: 3.20.2
   LATEST_STABLE_PYTHON: 3.14
-  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']"
 
 jobs:
   check_changes:

.pre-commit-config.yaml

@@ -26,6 +26,6 @@ repos:
     hooks:
     - id: black
 -   repo: https://github.com/pycqa/flake8
-    rev: 3.9.2
+    rev: 3.9.2 # version-scanner: ignore
     hooks:
     - id: flake8

ci/run_conditional_tests.sh

@@ -17,7 +17,7 @@
 # `BUILD_TYPE` should be one of ["presubmit", "continuous"]
 # `TEST_TYPE` should be one of ["docs", "docfx", "prerelease", "unit"]
 # or match the name of the nox session that you want to run.
-# `PY_VERSION` should be one of ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+# `PY_VERSION` should be one of ["3.10", "3.11", "3.12"]
 
 # `TEST_TYPE` and `PY_VERSION` are required by the script `ci/run_single_test.sh`
 

ci/run_single_test.sh

@@ -16,7 +16,7 @@
 
 # This script requires the following environment variables to be set:
 # `TEST_TYPE` should be one of ["lint", "lint_setup_py", "docs", "docfx", "prerelease"]
-# `PY_VERSION` should be one of ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+# `PY_VERSION` should be one of ["3.10", "3.11", "3.12", "3.13"]
 
 # This script is called by the `ci/run_conditional_tests.sh` script.
 # A specific `nox` session will be run, depending on the value of 
@@ -70,14 +70,6 @@ case ${TEST_TYPE} in
         ;;
     unit)
         case ${PY_VERSION} in
-        "3.7")
-            nox -s unit-3.7
-            retval=$?
-            ;;
-        "3.8")
-            nox -s unit-3.8
-            retval=$?
-            ;;
         "3.9")
             nox -s unit-3.9
             retval=$?

packages/gapic-generator/docs/getting-started/_verifying.rst

@@ -8,7 +8,7 @@ Create a virtual environment for the library:
 
 .. code-block:: shell
 
-  $ virtualenv ~/.local/client-lib --python=`which python3.7`
+  $ virtualenv ~/.local/client-lib --python=`which python3.10`
   $ source ~/.local/client-lib/bin/activate
 
 Next, install the library:

packages/gapic-generator/docs/getting-started/bazel.rst

@@ -29,7 +29,7 @@ install it is simply downloading the binary and making it executable:
 Python and Dependencies
 ~~~~~~~~~~~~~~~~~~~~~~~
 Bazel build is mostly hermetic, with a few exceptions for Python generator.
-Specifically it expects Python 3.7+ with the python dev packages to be installed.
+Specifically it expects Python 3.10+ with the python dev packages to be installed.
 
 On Linux, to install those, simply run:
 

packages/gapic-generator/docs/getting-started/index.rst

@@ -2,7 +2,7 @@ Getting Started
 ---------------
 
 This code generator is implemented as a plugin to ``protoc``, the compiler
-for `protocol buffers`_, and will run in any environment that Python 3.7+ and
+for `protocol buffers`_, and will run in any environment that Python 3.10+ and
 protocol buffers do.
 
 It is recommended to install the tool locally and run it through ``protoc``.