Columns that are assigned a `PENDING_COMMIT_TIMESTAMP()` value cannot be
included in `THEN RETURN` clauses.

Fixes this build error:
https://github.com/googleapis/python-spanner-sqlalchemy/actions/runs/19180600096/job/54948361746?pr=798

A recent fix in the Emulator caused the samples for SQLAlchemy to start
fail. That is not due to a bug in the Emulator, but due to a bug __fix__
in the Emulator. Previously, the Emulator would allow something that
would fail on Spanner. That difference has now been fixed.

Fixes #797

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

Towards googleapis/librarian#2460

---------

Co-authored-by: Knut Olav Løite <koloite@gmail.com>
Co-authored-by: ohmayr <omairn@google.com>

This PR adds support for Python 3.14 to the library.

Key changes include:
- Updating `setup.py` to include the Python 3.14 classifier.
- Updating `noxfile.py`:
   - Add 3.14 to test sessions.
- Refactor to use version constants (`UNIT_TEST_PYTHON_VERSIONS`,
`SYSTEM_TEST_PYTHON_VERSIONS`).
- Update `DEFAULT_PYTHON_VERSION` and
`DEFAULT_PYTHON_VERSION_FOR_SQLALCHEMY_20` to
"3.14".
   - Update `BLACK_VERSION` to `23.7.0` for Python 3.14 compatibility.
- Updating `.github/workflows/test_suite.yml` to use appropriate Python
versions for each job including matrix strategies for `unit` and
`system` jobs.

NOTE: The following CI/CD check is failing:

* `SQLAlchemy Spanner dialect / compliance_tests_20`

Per Gemini: The `compliance_test_20` session in Nox is currently failing
on Python 3.13 and 3.14 due to issues with schema reflection for
elements with quoted names or mixed case. These appear to pre-existing
issues with the dialect's handling of such identifiers, rather than a
regressio introduced by Python 3.14.

The failing compliance test has been marked as not required so as to not
be a blocker for this PR. As noted in the issue that status should be
reverted when the issue is resolved.

Further details and investigation are tracked in Issue #805.

PR created by the Librarian CLI to initialize a release. Merging this PR
will auto trigger a release.

Librarian Version: v0.7.0
Language Image:
us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:c8612d3fffb3f6a32353b2d1abd16b61e87811866f7ec9d65b59b02eb452a620
<details><summary>sqlalchemy-spanner: 1.17.2</summary>

##
[1.17.2](v1.17.1...v1.17.2)
(2025-12-15)

### Bug Fixes

* Retrieve columns in compound indexes in correct order (#798)
([9afe49b](9afe49bb))

</details>