Drop older Python support / increase min required Python version

[albertz]

I just want to open this issue to discuss this, and also on what version we want to increase to.

On RETURNN, currently we require Python 3.7. See rwth-i6/returnn#487 for some discussion and also what features are available since Python 3.7 (or closeby versions). But there is also another issue to maybe increase this further (rwth-i6/returnn#1326).

Python >=3.8 is required for i6_core: rwth-i6/i6_core#409, rwth-i6/i6_core#408

Python >=3.8 is also required for i6_models: rwth-i6/i6_models#11.