Take translation out of the build process

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Can't compile translation catalog when installing from source #342
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• 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?

• Compiled french translations are now kept within the repo and package. No need to make translate on each clone of the source.
• A new Makefile entry make checkfrench ensures that the french catalog is up-to-date AND that the compiled version is older than the catalog. See notes.
• The linting CI runs this checkfrench so a PR adding translatable messages cannot be merged if the french translation is not given.

Does this PR introduce a breaking change?

No.

The only change is for developers. make translate needs only to be run if a translatable message was added, after make findfrench was run and the "xscen.po" file updated with the french translation. So, exactly what the contributing guide says to do.

Other information:

The date check on mo files is weak : a catalog file could have a modified time changed even tough no real change was done. And (in theory) a compiled file could have a recent date even though it is not in sync wit the catalog. However, binary comparion of two mo files taken from the same catalog fails. My guess is that the compilation time is included in the mo file, and I didn't want to dig deeper. We don't update the translation catalogs often, so this should be enough.

@Zeitsperre This removes the setup.py stuff. Feel free to change the build system if you feel like doing so!

[aulemahal]

As seen on the failure of commit aaee623, the new CI element works as expected.

[aulemahal]

And I checked: installing from this branch with "pip git+https://..." works, while it does not for main.

[Zeitsperre]

Well done!

[Zeitsperre]

@aulemahal Do we want to enable the French RTD documentation in this PR as well?

[aulemahal]

Je ne pense pas qu'il y ait un besoin et je ne suis personnellement pas à tout à fait à l'aise avec une traduction automatique.