Migrate away from Python 3.6

[mambelli]

DE 2.x reference platform is Alma9 w/ Python 3.9. We want to migrate the main checks/operations to that platform (RPM building, unit tests, and linting). Would be nice to keep also checks on some other platform (to verify compatibility, different checks may find different problems...). Anyway, python 3.6 is no more supported because it is deprecated and libraries are not supporting it in the current versions and should be removed. The minimum is Python 3.7.
Not exhaustive list of things to do:

• update setup.py
• update containers
• check the code for incompatibilities w/ new library versions
• make sure all tests in decisionengine and decisionengine_modules are successful