Improve detection of s3fs and skip tests requiring it

[avalentino]

No description provided.

[ap--]

Hello @avalentino

could you clarify under what circumstances this is important?
Running the test suite via nox -s tests pulls in s3fs

[avalentino]

Hello @avalentino

could you clarify under what circumstances this is important? Running the test suite via nox -s tests pulls in s3fs

I maintain the Debian package for universal_pathlib.
The build process is done in isolation (no access to the internet) and the test suite is run regularly at each update of the package.
Without a pytest mark we would have test errors or would be forced to exclude tests one by one.

When the patch was submitted s3fs was not available in Debian.
Many of the test using it are already automatically skipped if s3fs is missing.
There are few ones for which this does not happen and those are addressed by this PR.