Add mypy type checking

[mdboom]

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[codecov-io]

Codecov Report

Merging #7 into master will decrease coverage by 0.9%.
The diff coverage is 89.71%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   92.78%   91.87%   -0.91%     
==========================================
  Files          14       14              
  Lines        3062     3115      +53     
==========================================
+ Hits         2841     2862      +21     
- Misses        221      253      +32

Impacted Files	Coverage Δ
pytoshop/codecs.py	95.53% <100%> (+0.02%)	⬆️
pytoshop/core.py	92.36% <100%> (ø)	⬆️
pytoshop/__init__.py	100% <100%> (ø)	⬆️
pytoshop/user/nested_layers.py	89.34% <80.55%> (-1.03%)	⬇️
pytoshop/image_data.py	89.88% <81.81%> (-6.27%)	⬇️
pytoshop/util.py	93.96% <83.33%> (-0.73%)	⬇️
pytoshop/color_mode.py	93.33% <83.33%> (-2.97%)	⬇️
pytoshop/layers.py	93.6% <85.71%> (-0.96%)	⬇️
pytoshop/tagged_block.py	89.96% <90.32%> (-2.17%)	⬇️
pytoshop/blending_range.py	96.06% <92.85%> (-0.72%)	⬇️
... and 2 more

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