Fix dependency manifest and CI

[sisp]

I've fixed the dependency manifest to install isort only for Python <4. The reason is this: Since this project requires Python 3.6+, only dependencies that also require Python 3.6+ or lower can be resolved. The last version of isort that still required only Python 3.6+ used an upper bound (<4) on the Python version specifier, so isort cannot be resolved anymore. To work around this problem, I've added the Python version upper bound only to isort.

I've also fixed the CI config to use Ubuntu 20.04 when testing against Python 3.6 because that's the last Ubuntu version supporting Python 3.6. CI checks for all other Python versions run against the latest available Ubuntu version.

These changes should restore the installation with Poetry and CI. :crossed_fingers:

[timofurrer]

@sisp thank you for the contribution!

I wonder if we should just lift the Python minimum requirement to the latest still supported Python version instead.

WDYT?

[sisp]

You're welcome.

I guess we could. An EOL Python interpreter shouldn't be used anymore, so it should be fine to discontinue supporting it. How about releasing v0.3.1 first still with Python 3.6+ support which includes the upper bound removal fix? Then, we can drop Python 3.6 and 3.7, also extend the test matrix to Python 3.11 and 3.12, and release v0.4.0?

[timofurrer]

@sisp yes, can do!

Would you be open to contribute the update of the interpeters, too?

[sisp]

Yes, gladly.