Drop older Python support, require Python >=3.8, TF >=2

[albertz]

Via #487, we are now supporting Python >=3.7.

Do we want to bump this and require Python >=3.8 or later? See also the Python feature list in #487.

The two main new features this gives us:

• typing.Protocol requires Python >=3.8.

• Slash (/) in function argument list to mark positional-only arguments (doc) requires Python >=3.8.

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

Same discussion for Python >=3.8 for i6_models: rwth-i6/i6_models#11

[christophmluscher]

I would say at least 3.8. Ubuntu 22 defaults to 3.10, so maybe move to 3.10?!

[curufinwe]

3.8 is ok (as it's the default for ubuntu 20.04). Anything higher I would delay for now until we (Apptek) have firmly tested our new ubuntu 22.04 images. Thanks!

[albertz]

Btw, I think we don't really have any place where we explicitly write the min required Python version? We should maybe add that to the README or so.

[NeoLegends]

Do you plan to make any changes to RETURNN during the version bump (other than bumping the min version)? I don't see many changes in 3.7 vs. 3.8 that would warrant large code changes? Maybe some arguments can be made positional-only for better forwards-compatibility, but this is at the same a backwards-compatibility risk, so I'm not sure it should be done to any existing code.

[albertz]

Do you plan to make any changes to RETURNN during the version bump

No, of course not. This would be unrelated. Also not really necessary, not important at all.

[NeoLegends]

Ah, by dropping support for 3.7 we also drop support for TF1. See #1668 (comment). TF1 has been EOL for quite some time now, so I believe it's ok to drop support given that you can always use an older RETURNN version for your legacy-TF experiments as well.