Reenable py310 and add support policy

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• 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?

• Re-enables support for Python3.10.
• Adds a support policy page to the documentation explaining our reasoning behind dropping/supporting new versions.

Does this PR introduce a breaking change?

No, in fact it reverts a breaking change.

Other information:

Internal discussions among the development team had us wondering if we were maybe too quick to drop Python3.10. It's possible that we will drop it once xarray terminates support (perhaps by summer, 2025).

The support policy could be adapted to multiple projects. Might be a good idea for posterity.

[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 (v2025.1.8).

No further action is required.

[aulemahal]

Agreed, but I would promise even less support for non-spec0 python versions.