[sflow][port_index_mapper.py] Convert to Python 3; Use logger from sonic-py-common for uniform logging

[jleveque]

- Why I did it

As part of moving all SONiC code from Python 2 (no longer supported) to Python 3

- How I did it

• Convert port_index_mapper.py script to Python 3
• Use logger from sonic-py-common for uniform logging
• Reorganize imports alphabetically per PEP8 standard
• Two blank lines precede functions per PEP8 standard

- How to verify it

Ensure port_index_mapper.py still functions correctly

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

[jleveque]

Closing in favor of #5800, which fixes bugs and now also converts to Python 3