feat: allow use with importlib_metadata version 4

[Neki]

This change makes single-source compatible with more packages, as other
packages may require importlib_metadata version 4.

The only breaking change in version 4 is:

PackageMetadata as returned by metadata() and Distribution.metadata() now provides normalized metadata honoring PEP 566

which has no impact on single-source (from https://github.com/python/importlib_metadata/blob/main/CHANGES.rst).

[Neki]

Travis CI doesn't seem to run on this branch, but I ran the tests with pytest with the latest version and everything passed:

% poetry update
...
% pytest
============================================================================ test session starts =============================================================================
platform linux -- Python 3.8.10, pytest-7.0.1, pluggy-1.0.0
rootdir: /home/benoit/terality/single-source
plugins: mock-3.6.1
collected 15 items

tests/test_regex.py .........                                                                                                                                          [ 60%]
tests/test_version.py ......                                                                                                                                           [100%]

============================================================================= 15 passed in 0.04s =============================================================================

[rabbit72]

That's a good idea. Thank you for contribution.

[rabbit72]

It took longer to release due to my vacation. I have to migrate from travis ci I think