Remove final python2 pieces

[tovrstra]

Fixes #8

[codecov]

Codecov Report

Merging #38 into master will decrease coverage by 0.49%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #38     +/-   ##
=========================================
- Coverage   98.05%   97.56%   -0.5%     
=========================================
  Files          18       18             
  Lines        3035     2870    -165     
  Branches     1796      262   -1534     
=========================================
- Hits         2976     2800    -176     
  Misses         31       31             
- Partials       28       39     +11

Impacted Files	Coverage Δ
python-cellcutoff/cellcutoff/ext.pyx	75.42% <ø> (ø)	⬆️
python-cellcutoff/cellcutoff/__init__.py	100% <100%> (ø)	⬆️
cellcutoff/decomposition.h	86.95% <0%> (-13.05%)	⬇️
cellcutoff/sphere_slice.cpp	95.77% <0%> (-2.9%)	⬇️
cellcutoff/tests/test_usage.cpp	98.5% <0%> (-1.5%)	⬇️
cellcutoff/iterators.cpp	92.38% <0%> (-0.42%)	⬇️
cellcutoff/tests/test_cell.cpp	99.62% <0%> (-0.15%)	⬇️
cellcutoff/cell.cpp	95.8% <0%> (-0.15%)	⬇️
cellcutoff/tests/test_sphere_slice.cpp	98.78% <0%> (-0.08%)	⬇️
cellcutoff/tests/test_iterators.cpp	100% <0%> (ø)	⬆️
... and 4 more

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