Drop python 3.7 support

[SkafteNicki]

What does this PR do?

Python 3.7 is blocking some other PRs:
#1636
#1570
and since the security support for 3.7 is soon ending, I think we can drop it.

Additionally, we are soon going to add python 3.11 support:
#1613

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1640 (0f2fcaf) into master (cb9540f) will decrease coverage by 36%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1640     +/-   ##
========================================
- Coverage      89%     53%    -36%     
========================================
  Files         228     228             
  Lines       12796   12794      -2     
========================================
- Hits        11363    6770   -4593     
- Misses       1433    6024   +4591     

[Borda]

let's wait for #1613
resolving issue of mentioned PRs in their scope #1636 and #1570 :)

TODO:

• bump sklearn version
• remove skipif for sklearn

[SkafteNicki]

@Borda now the only problem that seems to remain is that the docker image used for the "old" build on azure still uses python 3.7 and not 3.8.
There is a weird downgrade in the 1.9 image reported here:
pytorch/pytorch#60932
and here is a full list of python version for each image:
pytorch/pytorch#73714 (comment)