Fix lazy indexing

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Lazy indexing not lazy #1483
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.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?

• lazy_indexing will drop any auxiliary coords it find on the intermediate array that was triggering computation. The output is not affected.
• New xclim.testing.helpers.assert_lazy. A context manager to ensure the code block is not triggering any computation. I might take time to add it to other tests in another PR.
• Took the opportunity to fix a deprecation warning coming out of importlib.resources. The way we opened and searched for module data files was deprecated. Oups, I realized this deprecation concerns python >= 3.9, but we still support python 3.8. This change doesn't justify pinning a new python.

Does this PR introduce a breaking change?

No.

Other information:

[github-actions]

Warning
This Pull Request is coming from a fork and must be manually tagged approved in order to perform additional testing.

[Zeitsperre]

Neat!