[MISC] Fix Python 3.12 compatibility

[sinclert-canonical]

This PR fixes Python 3.12 compatibility after the pathlib.Path family of classes changed in order to tweak how they behave when they are are extended via sub-classes.

References

• Python 3.12 release notes.

Additional changes

• Update data-integrator charm base to fix integration tests

[carlcsaposs-canonical]

I think we should avoid touching this until we add support for 24.04

and at that point, I think we should be using https://pypi.org/project/charmlibs-pathops/ instead of a workaround

[sinclert-canonical]

Summary from online meeting:

• @carlcsaposs-canonical is worried that these changes are not enough to get the support ubuntu@24.04 base. Further validation is needed.
• For local set-ups, there is nothing stopping folks to run code / tests using Python 3.12+, as denoted by this line within the pyproject.toml file. We should change that to make it clear that Python 3.12+ is unsupported.