hard check macOS version for feature settings

[typemytype]

see #283

[codecov]

Codecov Report

Merging #335 into master will decrease coverage by 0.1%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
- Coverage   67.98%   67.88%   -0.11%     
==========================================
  Files          70       70              
  Lines        7281     7283       +2     
==========================================
- Hits         4950     4944       -6     
- Misses       2331     2339       +8

Impacted Files	Coverage Δ
drawBot/context/baseContext.py	76.82% <66.66%> (-0.04%)	⬇️
drawBot/context/tools/openType.py	60.49% <0%> (-8.65%)	⬇️

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