Fix to get regex to work on python > 3.6

[nicochidt]

If we try to load python-whois on python3.7 it fails with the following traceback:

/bin/pwhois netflix               [12:53:30]
Traceback (most recent call last):
  File "/lib/python3.7/sre_parse.py", line 1021, in parse_template
    this = chr(ESCAPES[this][1])
KeyError: '\\s'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/bin/pwhois", line 3, in <module>
    import argparse, pythonwhois, json, datetime, sys
  File "/lib/python3.7/site-packages/pythonwhois/__init__.py", line 1, in <module>
    from . import net, parse
  File "/lib/python3.7/site-packages/pythonwhois/parse.py", line 363, in <module>
    registrant_regexes = [preprocess_regex(regex) for regex in registrant_regexes]
  File "/lib/python3.7/site-packages/pythonwhois/parse.py", line 363, in <listcomp>
    registrant_regexes = [preprocess_regex(regex) for regex in registrant_regexes]
  File "/lib/python3.7/site-packages/pythonwhois/parse.py", line 205, in preprocess_regex
    regex = re.sub(r"\\s\*\(\?P<([^>]+)>\.\+\)", r"\s*(?P<\1>\S.*)", regex)
  File "/lib/python3.7/re.py", line 192, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/lib/python3.7/re.py", line 309, in _subx
    template = _compile_repl(template, pattern)
  File "/lib/python3.7/re.py", line 300, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/lib/python3.7/sre_parse.py", line 1024, in parse_template
    raise s.error('bad escape %s' % this, len(this))
re.error: bad escape \s at position 0

It is solved by adding the proper scape sequence in the regex expresion.

[mewforest]

It have to be merged, because it fixes library on latest Python, i.e. it works now in Python 3.8.