Fix DeprecationWarnings in newer Pythons (3.6+)

[mcarans]

I see these warnings each time I run:

...python3.6/site-packages/slugify/main.py:65
  ...python3.6/site-packages/slugify/main.py:65: DeprecationWarning: invalid escape sequence \p
    '''

...python3.6/site-packages/slugify/main.py:98
  ...python3.6/site-packages/slugify/main.py:98: DeprecationWarning: invalid escape sequence \L
    PRETRANSLATE = re.compile(u'(\L<options>)', options=convert_dict)

...python3.6/site-packages/slugify/main.py:140
  ...python3.6/site-packages/slugify/main.py:140: DeprecationWarning: invalid escape sequence \p
    unwanted_chars_re = u'[^\p{{AlNum}}{safe_chars}]+'.format(safe_chars=re.escape(self._safe_chars or ''))

...python3.6/site-packages/slugify/main.py:144
  ...python3.6/site-packages/slugify/main.py:144: DeprecationWarning: invalid escape sequence \p
    unwanted_chars_and_words_re = unwanted_chars_re + u'|(?<!\p{AlNum})(?:\L<stop_words>)(?!\p{AlNum})'

Perhaps this is the problem: https://stackoverflow.com/questions/50504500/deprecationwarning-invalid-escape-sequence-what-to-use-instead-of-d