Tighten up regexes
Tighten up regexes

 - Match the simpler strchr()y .config parsing done by the C
   implementation

 - Spell out \w as [a-zA-Z0-9_]. Easier to verify.

 - Use ASCII mode for Python 3 to be consistent with Python 2,
   where it's already the default. \s no longer matches obscure Unicode
   stuff.

This also speeds up regex matching during parsing by about 15% on Python
3, increasing parsing performance by a few %. Looks like there's a tiny
improvement for Python 2 as well.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib