Fix deprecation warning regarding invalid escape sequences.

[tirkarthi]

Fixes below warning by using https://github.com/asottile/pyupgrade/

 find . -iname '*.py' | grep -vE 'example|docs' | xargs -P 4 -I{} python3.8 -Wall -m py_compile {}
./polyglot/text.py:218: DeprecationWarning: invalid escape sequence \w
  tokens = nltk.tokenize.regexp_tokenize(self.raw, "\w*('\w*)+|\w+|[^\w\s]|\s")