Remove py3.5 and py3.6 support and deprecated methods.

[jjnicola]

No description provided.

[bjoernricks]

The linter is complaining about some unused imports. You can raise the minimum Python version to 3.7 if you want. I don't see a reason why we should support Python 3.6. But that's up to you.

[codecov]

Codecov Report

Merging #316 into master will increase coverage by 0.18%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   73.35%   73.54%   +0.18%     
==========================================
  Files          23       23              
  Lines        2560     2529      -31     
==========================================
- Hits         1878     1860      -18     
+ Misses        682      669      -13     

Impacted Files	Coverage Δ
ospd/logger.py	0.00% <ø> (ø)
ospd/server.py	31.41% <0.00%> (+0.39%)	⬆️
ospd/vts.py	97.16% <ø> (+0.90%)	⬆️
ospd/ospd.py	71.15% <100.00%> (+0.01%)	⬆️
ospd/parser.py	91.66% <100.00%> (+3.03%)	⬆️

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