End Python 2.7 support

[hoefling]

Python 2.7 has reached EOL on 1st Jan 2020, and with the CI services not offering the necessary infrastructure, supporting it has got very tedious. This PR removes 2.7 from the CI pipeline.

[codecov-commenter]

Codecov Report

Merging #167 (c147027) into master (6ed9d42) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
- Coverage   88.41%   88.25%   -0.17%     
==========================================
  Files          12       12              
  Lines        2081     2044      -37     
==========================================
- Hits         1840     1804      -36     
+ Misses        241      240       -1     

Impacted Files	Coverage Δ
src/main.c	77.77% <0.00%> (-5.56%)	⬇️
src/tree.c	95.32% <0.00%> (-0.13%)	⬇️
src/template.c	90.36% <0.00%> (-0.07%)	⬇️
src/constants.c	96.32% <0.00%> (-0.04%)	⬇️
src/platform.h	100.00% <0.00%> (+16.00%)	⬆️

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