fix the CI

[DahanDv]

I'm sorry in advance that this comes without an issue
It seems this has been discussed in some of the latest issues, and there's no point in opening another one

Fixing the CI:

• adding py versions up to 3.12
• to keep support for 3.6, a few steps had to be taken:
  • The flake test runs with 3.7 due to Flake's unresolved dependency issue.
  • Flake bumped to the highest version that can run on 3.7 (instead of the latest, which is not compatible with py 3.7)
  • due to a lack of image for py 3.6 with ubuntu-latest, the unit test for py 3.6 runs in a dedicated job on ubuntu 20.04

[Naramsim]

Thanks a lot!