Testing of MkDocs-Macros and compatibility with Python 3.8 and 3.9

[fralau]

A test suite was implemented, using pytest and MkDocs-Test.

With it, we can test that additions or corrections to the plugin work, while keeping the integrity of the existing code base.

Thanks to Github workflows, each new push triggers a test on the following versions of Python:
3.8, 3.9, 3.10, 3.11, 3.12.

⚠️ Warning: Python 3.8 and 3.9 have both reached end of life (source). In view of the fact that MkDocs-Macros is used by many websites, we will keep supporting it in new releases, as long as it doesn't break because dependencies are no longer supported.
In any case, you should migrate to at least Python 3.10 as soon as possible.