feat: Add support for Python 3.14 (#976)

This PR adds support for Python 3.14 to the library.

   Key changes include:
- Adding Python 3.14 to the test matrix in
`.github/workflows/unittest.yml`.
   - Updating `setup.py` to include the Python 3.14 classifier.
   - Adding `testing/constraints-3.14.txt`.
- Updating `noxfile.py` to include 3.14 sessions, and set default to
3.14.
- Updating `CONTRIBUTING.rst` to list Python 3.14 as a supported
version.

NOTE: The Kokoro Samples 3.14 check will fail. The docker container for
running that session does not yet have Python 3.14 in it. That is being
done in parallel. The fact that the Samples check does not pass should
not prevent this from being merged.
NOTE: tweaked a set of tests to no longer use pytz which was a transient
dependency in the past but the library that used to install with it no
longer does. Moved timezone info from pytz to the standard library's
timezone module.

---------

Co-authored-by: ohmayr <omairn@google.com>
Co-authored-by: Tim Sweña (Swast) <swast@google.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Chalmer Lowe <chalmerlowe@google.com>

Author: 5 people

.github/workflows/lint.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v5
       with:
-        python-version: "3.10"
+        python-version: "3.14"
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel

.github/workflows/unittest.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python: ['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
@@ -45,7 +45,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v5
       with:
-        python-version: "3.10"
+        python-version: "3.14"
     - name: Install coverage
       run: |
         python -m pip install --upgrade setuptools pip wheel

.kokoro/presubmit/system-3.13.cfg .kokoro/presubmit/system-3.14.cfg.kokoro/presubmit/system-3.13.cfg renamed to .kokoro/presubmit/system-3.14.cfg

@@ -3,5 +3,5 @@
 # Only run the following session(s)
 env_vars: {
   key: "NOX_SESSION"
-  value: "system-3.13"
+  value: "system-3.14"
 }