feat: Write CACHEDIR.TAG file

[neilramsay]

Write a Cache Directory Tag file "CACHEDIR.TAG" to created directory.

The CACHEDIR.TAG file is used by various tools to mark a directory as cache, so that it can be handled differently. Some backup tools look for this file to exclude the directory. For example, tar includes the --exclude-caches option.

See https://bford.info/cachedir/ for more details.

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• 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

[neilramsay]

Hi @gaborbernat,
I've been having a look at the build error logs, and I'm seeing errors in placed I wouldn't expect.
Such as test_creator.py::test_destination_exists_file or test_creator.py::test_os_path_sep_not_allowed.

Do you have any guidance on how I can be helpful?

[neilramsay]

I think the current failures are due to the current working directory of tox relative to where I'm expecting .venv to be.
I'll push something up in a few hours.

[gaborbernat]

Ok, when the CI passes we can merge it.

[neilramsay]

Thanks @gaborbernat for guiding me through this PR, and the improvements shown in the PR Feedback commit.

[gaborbernat]

Thank you very much for your contribution.