Support for fs.modified

[benrutter]

Currently, calling fs.modified(filepath) throws a NotImplementedError. It's a really quick add since this information is already available from fs.info(filepath).

This PR adds in modified and _modified as methods to the filesystem as a convenience.

Note: I've added a test that I think should work, but I can't actually get my environment setup to correctly run the test suite (I couldn't find a contributor guide but appologies if I missed something obvious). When I try to run the tests in python 3.10 I get a very strange error on importing paramiko because it uses asnyc as a keyword argument somewhere under the hood which is forbidden.

[shcheklein]

@benrutter could you please install pre-commit and run linters?

(in you virtual env of similar):

pip install pre-commit
pre-commit install
git reset --soft HEAD^1
git add <your files>
git commit <your commit info>

do git add + git commit twice if needed (it can fix the style for you the first time you run it).

otherwise, looks good to me, but let's see what @efiop and @skshetry say

[benrutter]

Thanks for taking the time to review @shcheklein 😁 I've reran the reprecommit hooks and overwritten the commit. Looks like linters are all happy now

[shcheklein]

@efiop please merge if it looks good (I don't have maintainer access to this repo)