Make test suite pass with pytest>=7.0

[pjbull]

In #202 we added a dev requirement to use pytest<7.0 since the test suite was breaking with the latest pytest version.

Looks like a compatibility issue with pytest-cases:
smarie/python-pytest-cases#251

Here's the stack trace:

__________________________________________________________________________________________________________________________________________________________________ ERROR collecting tests/test_s3_specific.py __________________________________________________________________________________________________________________________________________________________________
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pluggy/_hooks.py:265: in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pluggy/_manager.py:80: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/_pytest/python.py:257: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/_pytest/python.py:487: in _genfunctions
    fixtures.add_funcarg_pseudo_fixture_def(self, metafunc, fm)
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/_pytest/fixtures.py:164: in add_funcarg_pseudo_fixture_def
    if not metafunc._calls[0].funcargs:
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pytest_cases/plugin.py:931: in __getitem__
    return self.calls_list[item]
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pytest_cases/plugin.py:942: in calls_list
    self.create_call_list_from_pending_parametrizations()
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pytest_cases/plugin.py:994: in create_call_list_from_pending_parametrizations
    remove_empty_ids(callspec)
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/pytest_cases/fixture_parametrize_plus.py:556: in remove_empty_ids
    callspec._idlist = [c for c in callspec._idlist if not c.startswith(EMPTY_ID)]
../../miniconda3/envs/cloudpathlib/lib/python3.8/site-packages/attr/_make.py:626: in _frozen_setattrs
    raise FrozenInstanceError()
E   attr.exceptions.FrozenInstanceError

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Originally posted by @jayqi in #202 (comment)