Issue #9036: Throughout the code base, Py_CHARMASK is used on 8-bit wide

Issue #9036: Throughout the code base, Py_CHARMASK is used on 8-bit wide
signed/unsigned chars or on integers directly derived from those. In all
cases, it could be replaced by a simple cast to (unsigned char). Reasons
for the change:

  a) Make the comment more explicit.

  b) If char is unsigned, the cast is optimized away.

  c) If char is unsigned, gcc emits spurious "array subscript
     has type 'char'" warnings.