Prefix regexes with r to avoid 'invalid escape sequence' Deprecation warnings

[tomaarsen]

Hello!

Pull request overview

• Prefix some regexes in docstrings with r.

Details

See this GitHub Action for the source of these warnings:

nltk/tokenize/regexp.py::nltk.tokenize.regexp
  <doctest nltk.tokenize.regexp[2]>:1: DeprecationWarning: invalid escape sequence '\w'

nltk/tokenize/regexp.py::nltk.tokenize.regexp
  <doctest nltk.tokenize.regexp[4]>:1: DeprecationWarning: invalid escape sequence '\s'

nltk/tokenize/regexp.py::nltk.tokenize.regexp
  <doctest nltk.tokenize.regexp[6]>:1: DeprecationWarning: invalid escape sequence '\w'

nltk/tokenize/regexp.py::nltk.tokenize.regexp
  <doctest nltk.tokenize.regexp[11]>:1: DeprecationWarning: invalid escape sequence '\w'

nltk/tokenize/regexp.py::nltk.tokenize.regexp.RegexpTokenizer
  <doctest nltk.tokenize.regexp.RegexpTokenizer[0]>:1: DeprecationWarning: invalid escape sequence '\w'

This PR fixes these by prefixing r to these strings, turning them into "raw" strings, which is preferred for strings that represent regexes.

• Tom Aarsen