Handle byte strings in s:skip_special_chars

[blueyed]

Fixes #72.
Closes #71.
Closes #68.

[codecov]

Codecov Report

Merging #113 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #113   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files           1        1           
  Lines         207      207           
=======================================
  Hits          192      192           
  Misses         15       15

Impacted Files	Coverage Δ
indent/python.vim	92.75% <ø> (ø)	⬆️

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