Add support for Python 3.11 and 3.12, drop 3.7, and add test workflow

[WeisLeDocto]

Python 3.11 has been released more than a year ago, and Python 3.12 a few months ago.
However, there has not been any specific effort so far for ensuring that Crappy runs as expected on these version.
At the very least, Crappy should be built and tested on these versions.
The version tags on PyPI should then be updated accordingly.

Also, Python 3.7 now has the end-of-life status, and is not distributed with the recent Linux distributions.
Ubuntu 20.04 (April 2020) is distributed with Python 3.8, and Debian Bullseye (August 2021) is distributed with Python 3.9.
Windows 10 and 11 allow installing Python 3.7 to 3.12 directly from the Microsoft store.
Support for Python 3.7 can therefore reasonably be removed from Crappy.
The version tags on PyPI should then be updated accordingly.

After removing support for Python 3.7, the novelties added in Python 3.8 can be added to the code.
That includes the walrus operator, or the possibility to use keyword-only and positional-only arguments.

Ideally, the installation and import of Crappy on all the supported Python versions and platforms should be tested automatically in a GitHub workflow.

ToDo

• Test the compatibility of Crappy on Python 3.11 and 3.12
• Add features of Python 3.8, de facto removing support for Python 3.7
• Adjust the version tags accordingly on PyPI
• Add a GitHub workflow for testing the installation and import of Crappy on supported Python version and platforms