Update `RemoteStore.__str__` and add UPath tests

[d-v-b]

This PR changes the .__str__ method of RemoteStore to make it similar to LocalStore and MemoryStore (i.e., RemoteStore("s3://path").__str__ returns s3://path).

~I also add tests for UPath, and tweaked the test suite accordingly. The tests are failing, and I don't know why. @martindurant could you take a look if you have time?~ edit: tests are passing

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

[d-v-b]

the test failures were due to a trailing "/" character added to the url / path attributes in the "init-from-UPath" code paths, and that's resolved via str.rstrip('/')

[martindurant]

None of my comments are blockers