Establish supported Python version

[victorgarcia98]

There's still pending to define which minimum Python version we are going to support.

This should go in:

• setup.cfg
  • minimum version
  • classifiers
• Github Action Pipeline: Python version to use in testing.

FYI: In FlexMeasures, the minimum version that we support a.t.m. is Python 3.8.

[GustaafL]

I'm using the | or |= operator for adding dictionaries together which is supported since python 3.9. Have updated the setup.cfg to reflect that.