Fail to compile if CHAR_BIT != 8

[benjeffery]

Fixes #645

[codecov]

Codecov Report

Merging #667 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #667   +/-   ##
=======================================
  Coverage   87.75%   87.75%           
=======================================
  Files          23       23           
  Lines       17687    17687           
  Branches     3498     3498           
=======================================
  Hits        15522    15522           
  Misses       1062     1062           
  Partials     1103     1103           

Flag	Coverage Δ
#c_tests	89.03% <ø> (ø)
#python_c_tests	91.23% <ø> (ø)
#python_tests	98.99% <ø> (ø)

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