Deprecation and syntax warnings in Python 3.8

[tirkarthi]

Deprecation warning due to invalid escape sequences. Using raw strings or escaping them again helps in resolving this. Check https://github.com/asottile/pyupgrade/ for automatic fix of this.

find . -iname '*.py' | grep -Ev 'rdf4|doc|benchmark|tool' | xargs -P4 -I{} python3.8 -Wall -m py_compile {}
./slugify/__init__.py:84: DeprecationWarning: invalid escape sequence \s
  new = re.sub('[%s\s]+' % space_replacement, space_replacement, new)