test_entrypoints_backport and missing importlib_metadata

[stanislavlevin]

The test suite of numcodecs 0.13.1 started to fail (importlib_metadata is not installed) with:

==================================== ERRORS ====================================
_ ERROR collecting .run_venv/lib64/python3/site-packages/numcodecs/tests/test_entrypoints_backport.py _
.run_venv/lib64/python3/site-packages/numcodecs/tests/test_entrypoints_backport.py:10: in <module>
    if not importlib.util.find_spec("importlib_metadata").loader:  # pragma: no cover
E   AttributeError: 'NoneType' object has no attribute 'loader'

Note: if importlib_metadata is not installed (this case) the find_spec returns None.
https://docs.python.org/3/library/importlib.html#importlib.util.find_spec

importlib.util.find_spec(name, package=None)(https://docs.python.org/3/library/importlib.html#importlib.util.find_spec)
Find the spec for a module, optionally relative to the specified package name. If the module is in sys.modules, then sys.modules[name].spec is returned (unless the spec would be None or is not set, in which case ValueError is raised). Otherwise a search using sys.meta_path is done. None is returned if no spec is found.