Drop python<3.9, support modern libraries

[lopuhin]

In this PR we drop support for python 3.8 and older, as they are past their end of life. We add support to scikit-learn 1.x (testing on 1.6), and drop support for scikit-learn < 1.0

There are a few missing features left for future PRs:

• In this PR I'm disabling tests of gradcam explanation with Keras, as we required TF 1.x, but it does not support python 3.9+ so it's not breaking backwards compatibility as this did not work anyways. See updates Keras and tensorflow version [MNT] #47
• XGBoost starting from 2.0 fails some tests, but XGBoost 1.x works, so we document this and leave support for newer XGBoost for future PRs (and issue a warning if we detect an old XGBoost) -- this is also backwards compatible, see Support XGBoost 2.x and 3.x #49
• docs build has a lot of warnings, ideally we fix them and treat as errors, like before -- this is a blocker for next release

[lopuhin]

Updated description with the current state, this looks good to merge as the build is green and remaining issues were present previously.