Bump version to Python 3.10

[albertz]

Previous i6_core PR to bump Python version to 3.8: #409

The recent i6_core PR #633 introduced a Python 3.10 dependency for some parts, but not consistently.

E.g. missing are other config files (see the previous PR). And I would also suggest to explicitly mention that in the README or so somewhere.

If you think Python 3.10 is fine now, please can you also comment on the corresponding related issues?

• Issue to bump Python version in Sisyphus (to >3.8): Drop older Python support / increase min required Python version sisyphus#195
• Issue to bump Python version in RETURNN (to >3.8): Drop older Python support returnn#1720

(@curufinwe @NeoLegends @michelwi @JackTemaki)

[michelwi]

yes, bump to 3.10 seems reasonable to me