CONTRIBUTING: Drop Python 2 requirement

[holesch]

Not sure if this is correct, but it looks like scapy doesn't support Python 2 anymore (pyproject.toml has the Programming Language :: Python :: 3 :: Only classifier).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.55%. Comparing base (8f9c7a6) to head (f48814a).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4713      +/-   ##
==========================================
- Coverage   82.11%   78.55%   -3.56%     
==========================================
  Files         361      336      -25     
  Lines       86908    81701    -5207     
==========================================
- Hits        71365    64181    -7184     
- Misses      15543    17520    +1977     

see 288 files with indirect coverage changes

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[gpotter2]

Thanks for the PR & sorry for the delay