feat: Add Python 3.14 support and update CI

This commit adds support for Python 3.14 to the library, removes Python 3.7 and 3.8 from the nox sessions, and updates the GitHub Actions workflow.

The following changes were made:
- Added a 3.14 trove classifier in `setup.py`.
- Added a `grpcio` version constraint for Python 3.14 in `setup.py`.
- Added a 3.14 presubmit configuration file in `.kokoro/samples`.
- Added a `testing/constraints-3.14.txt` file.
- Updated the `CONTRIBUTING.rst` and `README.rst` files to include 3.14.
- Updated `owlbot.py` to include "3.14" in `unit_test_python_versions`.
- Updated `noxfile.py` to include "3.14" in `UNIT_TEST_PYTHON_VERSIONS` and the `prerelease_deps` session, and remove 3.7/3.8.
- Updated `.github/sync-repo-settings.yaml` to include 3.14.
- Updated `.github/workflows/unittest.yml` to include 3.14 and remove 3.7/3.8.
- Fixed a flaky test in `tests/unit/pubsub_v1/publisher/test_publisher_client.py`.
- Fixed an `AttributeError` in `google/cloud/pubsub_v1/subscriber/scheduler.py` that occurred in Python 3.14.
- Fixed `NameError` and `ValueError` bugs in `samples/snippets/publisher.py`.
- Removed Python 3.7 and 3.8 from `CONTRIBUTING.rst`, `README.rst`, and `owlbot.py`.
- Fixed a `ValueError` in the `prerelease_deps` nox session.

Author: google-labs-jules[bot]

.github/workflows/unittest.yml

@@ -5,13 +5,10 @@ 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
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
+        python: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
     steps:
     - name: Checkout
       uses: actions/checkout@v4

noxfile.py

@@ -233,12 +233,7 @@ def install_unittest_dependencies(session, *constraints):
 def unit(session, protobuf_implementation):
     # Install all test dependencies, then install this package in-place.
 
-    if protobuf_implementation == "cpp" and session.python in (
-        "3.11",
-        "3.12",
-        "3.13",
-        "3.14",
-    ):
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12", "3.13", "3.14"):
         session.skip("cpp implementation is not supported in python 3.11+")
 
     constraints_path = str(
@@ -448,12 +443,7 @@ def docfx(session):
 def prerelease_deps(session, protobuf_implementation):
     """Run all tests with prerelease versions of dependencies installed."""
 
-    if protobuf_implementation == "cpp" and session.python in (
-        "3.11",
-        "3.12",
-        "3.13",
-        "3.14",
-    ):
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12", "3.13", "3.14"):
         session.skip("cpp implementation is not supported in python 3.11+")
 
     # Install all dependencies
@@ -465,6 +455,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
     session.install(*system_deps_all)
 
+
     prerel_deps = [
         "protobuf",
         # dependency of grpc