Fix issue #7165 in _create_challenge_dirs()

[martin-c]

Some operating systems won't reliably raise EEXIST or EISDIR if a directory exists when filesystem.mkdir() is called, instead raising Read-only file system errors or similar even when path is in fact writable. To work around this we include a check to see if directory already exists. See Issue #7165 for more information.

Pull Request Checklist

• Edit the master section of CHANGELOG.md to include a description of
  the change being made.
• Add mypy type
  annotations
  for any functions that were added or modified.
• Include your name in AUTHORS.md if you like.

[adferrand]

Hello @martin-c, the PR almost LGTM! Could you merge/rebase master, fix the lint error and add a changelog entry? Then I will merge.

[martin-c]

webroot.py has been moved to certbot/_internal/plugins/webroot.py and I'm not sure how to apply my changes to the original file there. I will make a new pull request.