Faster encodedword unfold by 15x

[carsonip]

15x faster unfold function:

1. Use the re object's instance method instead of re.sub to skip _compile. See: https://stackoverflow.com/a/47477439/3315725
2. Change replacing backref to only replace part with empty string (I don't understand why it was done that way in the first place)
3. Use non-capturing groups for another 10-20% boost

In [22]: _RE_FOLDING_WHITE_SPACES = re.compile(r"(\n\r?|\r\n?)(\s*)")

In [23]: def unfold(value):
   ....:         """
   ....:         Unfolding is accomplished by simply removing any CRLF
   ....:         that is immediately followed by WSP.  Each header field should be
   ....:         treated in its unfolded form for further syntactic and semantic
   ....:         evaluation.
   ....:         """
   ....:         return re.sub(_RE_FOLDING_WHITE_SPACES, r'\2', value)
   ....: 

In [24]: %timeit unfold(x)
100000 loops, best of 3: 9.59 µs per loop

In [26]: _RE_FOLDING_WHITE_SPACES = re.compile(r"(?:\n\r?|\r\n?)")

In [27]: def unfold(value):
   ....:         """
   ....:         Unfolding is accomplished by simply removing any CRLF
   ....:         that is immediately followed by WSP.  Each header field should be
   ....:         treated in its unfolded form for further syntactic and semantic
   ....:         evaluation.
   ....:         """
   ....:         return _RE_FOLDING_WHITE_SPACES.sub('', value)
   ....: 

In [29]: %timeit unfold(x)
1000000 loops, best of 3: 598 ns per loop

[mailgun-ci]

Can one of the admins verify this patch?