Switch to using Python 3.8 instead of Python 3.7

[Innixma]

Python 3.7 is no longer supported by Python, and some frameworks such as AutoGluon only support Python 3.8+ in their latest releases. Further, many packages such as scikit-learn, pandas, numpy, etc. no longer support Python 3.7.

I'd recommend AMLB use Python 3.8 or Python 3.9 as the new default. (I've tested setting Python 3.9, works out of the box with AutoGluon).

[PGijsbers]

I think 3.9 would be a better choice as the scientific python community is about to drop releases for 3.8 in a month as per NEP 29. A secondary concern would be to provide support for installations of older frameworks, while the Python versions can be independent in principle, they are not in practice. I think it's now always fixed to be versions.python and there isn't a good mechanism for pinning Python versions based on framework versions. Suggestions welcome.

[Innixma]

I think 3.9 would be a better choice as the scientific python community is about to drop releases for 3.8 in a month as per NEP 29. A secondary concern would be to provide support for installations of older frameworks, while the Python versions can be independent in principle, they are not in practice. I think it's now always fixed to be versions.python and there isn't a good mechanism for pinning Python versions based on framework versions. Suggestions welcome.

3.9 should be fine, I actually switched to using 3.9 and it appears to work for AutoGluon (AutoGluon supports 3.8-3.10 in latest v0.7 release). Let me know if you'd like me to edit the PR to be 3.9, or else I think you should be able to directly commit to this PR and update it on your end.

[Innixma]

@PGijsbers Please feel free to close this PR in favor of #520