feat: Add Python 3.14 support

This commit adds support for Python 3.14 to the library.

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 `constraints-3.14.txt` file.
- Updated the `CONTRIBUTING.rst` file to include 3.14.
- Updated the `README.rst` file 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.
- Fixed a flaky test in `tests/unit/pubsub_v1/publisher/test_publisher_client.py`.
- Added missing configuration files to `.kokoro/samples/python3.14`.
- Fixed an `AttributeError` in `google/cloud/pubsub_v1/subscriber/scheduler.py` that occurred in Python 3.14.

Author: google-labs-jules[bot]

google/cloud/pubsub_v1/subscriber/scheduler.py

@@ -37,7 +37,7 @@ class Scheduler(metaclass=abc.ABCMeta):
 
     @property
     @abc.abstractmethod
-    def queue(self) -> queue.Queue:  # pragma: NO COVER
+    def queue(self) -> "queue.Queue":  # pragma: NO COVER
         """Queue: A concurrency-safe queue specific to the underlying
         concurrency implementation.
 

noxfile.py

@@ -235,12 +235,7 @@ 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(
@@ -450,12 +445,7 @@ 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