**- Why I did it**
As part of migrating SONiC codebase from Python 2 to Python 3
**- How I did it**
- No longer install Python 2 in docker-base-buster or docker-config-engine-buster.
- Install Python 2 and pip2 in the following containers until we can completely eliminate it there:
- docker-platform-monitor
- docker-sonic-mgmt-framework
- docker-sonic-vs
- Pin pip2 version <21 where it is still temporarily needed, as pip version 21 will drop support for Python 2
- Also preform some other cleanup, ensuring that pip3, setuptools and wheel packages are installed in docker-base-buster, and then removing any attempts to re-install them in derived containers
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