Follow semver: change dev version -> post

[gpotter2]

A very long time ago someone (can't find the issue) noted that we are using semver wrong (dev is BEFORE a release). They're actually right + it is useful to fix this as it allows someone to require a post release in their dependencies.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.00%. Comparing base (667dd94) to head (707ff34).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4976      +/-   ##
==========================================
+ Coverage   79.97%   80.00%   +0.03%     
==========================================
  Files         382      382              
  Lines       93995    93995              
==========================================
+ Hits        75169    75200      +31     
+ Misses      18826    18795      -31     

Files with missing lines	Coverage Δ
scapy/__init__.py	83.11% <100.00%> (ø)

... and 28 files with indirect coverage changes

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