Improve retry logic and add python 3 classifier

[blink1073]

No description provided.

[codecov]

Codecov Report

Merging #18 into master will decrease coverage by 1.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   91.43%   90.38%   -1.06%     
==========================================
  Files           3        3              
  Lines         257      260       +3     
==========================================
  Hits          235      235              
- Misses         22       25       +3     

Impacted Files	Coverage Δ
pytest_check_links/plugin.py	92.79% <0.00%> (-1.28%)	⬇️

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