Adopt Pytest v9.0+, RuntimeWarning changes

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Should we be showing numpy RuntimeWarnings? #2277
• 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?

• Modifies a few of the squelched RuntimeWarning messages in order to provide more information for users of indices, while silencing these messages for indicator users and the CLI.
• Updates pytest to the latest conventions (version 9.0+), enabling the new strict mode.

Does this PR introduce a breaking change?

For some developers, yes. There will likely be more noise emitted from workflows relying on xclim.indices. These can be set to be ignored using context managers:

with numpy.errstate(divide='ignore'):
    ...

or:

with warnings.catch_warnings():
    warnings.simplefilter("ignore")
    ...

Other information:

The new pytest now offers a strict mode. This means that configurations for markers and xfail must be explicitly defined, parametrizations must always be uniquely-defined, and the configuration found in pyproject.toml must not contain any errors. For more information: https://docs.pytest.org/en/stable/reference/reference.html#confval-strict

[aulemahal]

Looks good! Thanks!

[coveralls]

coverage: 92.101% (-0.005%) from 92.106%
when pulling 2422c74 on pytest-v9
into 8ce7a2c on main.