Remove python version checks

[aaronayres35]

part of #415

This PR removes any explicit python version checks and only keeps the code paths that are specific to python versions we plan to continue to support (python >=3.6).

[jwiggins]

A couple comments

[codecov-io]

Codecov Report

Merging #463 (be9d0ff) into master (4db18d2) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #463      +/-   ##
==========================================
- Coverage   30.33%   30.20%   -0.13%     
==========================================
  Files         206      206              
  Lines       18245    18185      -60     
  Branches     2466     2465       -1     
==========================================
- Hits         5534     5493      -41     
+ Misses      12371    12353      -18     
+ Partials      340      339       -1     

Impacted Files	Coverage Δ
kiva/fonttools/font.py	47.43% <ø> (+0.52%)	⬆️
enable/primitives/box.py	32.00% <0.00%> (-2.62%)	⬇️
enable/wx/constants.py	55.55% <0.00%> (-2.34%)	⬇️
enable/primitives/annotater.py	32.50% <0.00%> (-1.65%)	⬇️
enable/primitives/line.py	33.33% <0.00%> (-1.29%)	⬇️
enable/graphics_context.py	30.35% <0.00%> (-1.23%)	⬇️
enable/tools/apptools/command_tool.py	84.61% <0.00%> (-1.10%)	⬇️
enable/compass.py	29.26% <0.00%> (-0.86%)	⬇️
enable/canvas.py	26.59% <0.00%> (-0.78%)	⬇️
... and 54 more

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[jwiggins]

Thanks for the update