Support pytest 8

[jarrodmillman]

See #85. With pytest 8, we get

_________________________ ERROR collecting yaml2ics.py _________________________
/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/_pytest/runner.py:345: in from_call
    result: Optional[TResult] = func()
/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/_pytest/runner.py:390: in collect
    return list(collector.collect())
/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/_pytest/doctest.py:569: in collect
    module = import_path(
/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/_pytest/pathlib.py:590: in import_path
    raise ImportPathMismatchError(module_name, module_file, path)
E   _pytest.pathlib.ImportPathMismatchError: ('yaml2ics', '/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/yaml2ics.py', PosixPath('/home/runner/work/yaml2ics/yaml2ics/yaml2ics.py'))
=========================== short test summary info ============================
ERROR yaml2ics.py - _pytest.pathlib.ImportPathMismatchError: ('yaml2ics', '/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/yaml2ics.py', PosixPath('/home/runner/work/yaml2ics/yaml2ics/yaml2ics.py'))
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.22s ===============================