chore: update default Python version to 3.12 for tests and formatting

[tswast]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[tswast]

Owlbot is failing at the final step:

Step #5: noxfile.py.j2
Step #5: Traceback (most recent call last):
Step #5:   File "/workspace/python-bigquery-magics/owlbot.py", line 102, in <module>
Step #5:     s.shell.run(["nox", "-s", "format"], hide_output=False)
Step #5:   File "/synthtool/synthtool/shell.py", line 39, in run
Step #5:     raise exc
Step #5:   File "/synthtool/synthtool/shell.py", line 27, in run
Step #5:     return subprocess.run(
Step #5:   File "/usr/local/lib/python3.10/subprocess.py", line 524, in run
Step #5:     raise CalledProcessError(retcode, process.args,
Step #5: subprocess.CalledProcessError: Command '['nox', '-s', 'format']' returned non-zero exit status 1.
Finished Step #

I suspect this is because the gcr.io/cloud-devrel-public-resources/owlbot-python might not have Python 3.9+ installed? @parthea Do you know? I'm not seeing it in https://github.com/googleapis/testing-infra-docker/tree/main

[tswast]

Suggested change

	default_python_version="3.9",
	default_python_version="3.12",

[parthea]

Correct, only 3.8 and 3.10 are available in owlbot-python. We could update 3.8 to a newer runtime given that it's almost EOL

https://github.com/googleapis/synthtool/blob/master/docker/owlbot/python/Dockerfile

[parthea]

We could consider removing the automatic code formatting and add a presubmit to fail if the code is does not meet formatting requirements. Alternatively, code formatting could be outside of owlbot.py , via something like Github actions.

WDYT about removing the lines below?

python-bigquery-magics/owlbot.py

Lines 101 to 103 in 6c2ac31

	s.shell.run(["nox", "-s", "format"], hide_output=False)
	for noxfile in REPO_ROOT.glob("samples/**/noxfile.py"):
	s.shell.run(["nox", "-s", "blacken"], cwd=noxfile.parent, hide_output=False)