Ensure tox-created directories contain CACHEDIR.TAG

[akx]

This PR ensures that Tox-created work directories contain a CACHEDIR.TAG file, which can be used to exclude the directories from e.g. backups.

Fixes #3334

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

Why create these in sub folders? Why not just created it within the .tox root directory and be done with it?

With this implementation, the .tox directory itself won't contain the tag file, since there is no explicit mkdir call for the workdir. Only the subdirectories explicitly created by calls around Tox will be populated with the file.

Just because there isn't does not mean we can't add it.

[akx]

Why not just created it within the .tox root directory and be done with it?

I felt there was an idea behind the decision to not have an explicit call to create that directory – maybe that the workdir wouldn't be eagerly created before it's needed (just to hold a subdirectory).

Just because there isn't does not mean we can't add it.

Sure, I'll do that. Would you prefer eagerly creating the workdir with the cache tag, or having the places that create those workdir subdirs ensure the workdir has the cache tag?

[gaborbernat]

having the places that create those workdir subdirs ensure the workdir has the cache tag

Probably this 🤔

[akx]

having the places that create those workdir subdirs ensure the workdir has the cache tag

Probably this 🤔

It turned out a little trickier like that, since an user could point Tox to a pre-created workdir, which we then certainly do not want to litter with a CACHEDIR.TAG file.

I still didn't write tests for this incarnation, just in case you'd like to take a review look first.

[gaborbernat]

The CI is fialing

[gaborbernat]

Needs tests too 👍