Adopt SPEC0 calendar, drop Python3.9

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Adopt the Scientific Python SPEC0 Python calendar #1914
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Extends support for Python3.13 and drops support for Python3.9
• Raises the base required versions for a handful of dependencies based on Scientific Python's SPEC 0 standard:
  • ipython to v8.5.0+
  • matplotlib to v3.6.0+
  • numpy to v1.23+
  • scikit-learn to v1.1.0+
• Updates the codebase to use Python3.10+ conventions
• nbsphinx has been pinned to v0.9.5+

Does this PR introduce a breaking change?

Yes. Python3.9 support has been dropped.

Other information:

https://scientific-python.org/specs/spec-0000/
https://peps.python.org/pep-0619/

Notable changes:

• https://peps.python.org/pep-0604/
• https://peps.python.org/pep-0618/

[github-actions]

Note

It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2024.8.23).

No further action is required.

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[Zeitsperre]

Python3.13 builds require changes in coveralls: TheKevJames/coveralls-python#523

[Zeitsperre]

Will be performing a squash+merge to erase my mistakes not pollute the git history.

[coveralls]

coverage: 89.374% (-0.02%) from 89.394%
when pulling 24ac1ec on spec0-calendar
into e9ae422 on main.