Fix TCP retransmissions

[gpotter2]

fix https://github.com/secdev/scapy/issues/4197

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 82.17%. Comparing base (bcf500d) to head (94db327). Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4224      +/-   ##
==========================================
+ Coverage   82.12%   82.17%   +0.04%     
==========================================
  Files         350      350              
  Lines       83242    83269      +27     
==========================================
+ Hits        68364    68427      +63     
+ Misses      14878    14842      -36     

Files	Coverage Δ
scapy/layers/http.py	82.38% <100.00%> (+0.10%)	:arrow_up:
scapy/sessions.py	95.40% <55.55%> (-1.97%)	:arrow_down:

... and 14 files with indirect coverage changes