deps!: Drop support for Python 3.7 and 3.8 (AI Experiment) (#337)

* feat: Drop support for Python 3.7 and 3.8

Removes support for Python 3.7 and 3.8, establishing Python 3.9 as the new minimum supported version.

This change involves:
- Updating `python_requires` and classifiers in `setup.py`.
- Modifying Python versions in `noxfile.py` (default, unit tests, system tests) and ensuring constraint file logic remains correct.
- Updating the GitHub Actions workflow (`unittest.yml`) matrix, runner, and coverage job version.
- Deleting constraint files for Python 3.7 and 3.8 (`testing/constraints-3.7.txt`, `testing/constraints-3.8.txt`).
- Removing Kokoro sample configuration directories (`.kokoro/samples/python3.7/`, `.kokoro/samples/python3.8/`).
- Updating supported version mentions in `README.rst`.
- Removing 3.7 and 3.8 from the `ALL_VERSIONS` list in `samples/snippets/noxfile.py`.

* Updates python version in lint.yml

* Updates owlbot, removing reference to 3.8

* Updates CONTRIBUTING.rst

* updates pytest warnings

* Removes test_samples-impl ref to older virtualenv package

* Removes references to pandas older than 1.5.0

* Removes pandas older than 1.5 and misc changes

* updates pandas in setup.py

* more updates related to pandas

* still broken

* Updates FutureWarning tests to account for unittest coverage

* Updates json array type tests to account for unittest coverage

* updates python version checks to ensure coverage

* update json test for unittest coverage

* Update pandas_backports unittests to ensure coverage

* Updates per review comments

* moves class from version specific compliance file to generic file

* Removes weird cut and paste error

* fix linting errors

* updates import statement to ensure import of JSONArrowType

* Revise required github status checks

* update linting

* temporarily marking a class as no cover

* more updates

* marked several snippets as pragma no cover

* updates linting

* Updates constraints and setup.py

* migrates class from one time compliance file to another

* updating pyarrow version

* Updates linting

* removes determine all and module reload tests

* updates re: ndarrybackedextensionarray

* testing blacken as part of owlbot processing using 3.8

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* updates blacken to 3.10

* update python version in lint.yml

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* updates owlbot.py

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* updates owlbot.py

* testing lint.yml

* testing linting issue

* testing linting issue

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* updates lint_setup session

* Update noxfile.py

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: 3 people

bigquery/python-db-dtypes-pandas/snippets/requirements-test.txt

@@ -1,2 +1 @@
-pytest===7.4.4; python_version == '3.7'  # prevents dependabot from upgrading it
-pytest==8.3.3; python_version > '3.7'
+pytest==8.3.5