Fix cryptography deprecation warnings

[gpotter2]

Updates to use the decrepit module from cryptography when available (for 43+)

fixes #4508

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.68%. Comparing base (97a49f3) to head (27116ed).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/dot11.py	40.00%	3 Missing ⚠️
scapy/libs/rfc3961.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4516      +/-   ##
==========================================
- Coverage   81.68%   81.68%   -0.01%     
==========================================
  Files         355      355              
  Lines       84936    84940       +4     
==========================================
- Hits        69384    69383       -1     
- Misses      15552    15557       +5     

Files with missing lines	Coverage Δ
scapy/layers/kerberos.py	67.43% <100.00%> (ø)
scapy/libs/rfc3961.py	85.98% <66.66%> (ø)
scapy/layers/dot11.py	91.17% <40.00%> (-0.29%)	⬇️

... and 4 files with indirect coverage changes