Deprecation warning due to invalid escape sequences

[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|tool|doc' | xargs -P4 -I{} python3.8 -Wall -m py_compile {}
./tests/test_basic.py:62: DeprecationWarning: invalid escape sequence \$
  self.assertRegexpMatches(test, "alice:\$apr1\$")
./tests/test_basic.py:139: DeprecationWarning: invalid escape sequence \$
  self.assertRegexpMatches(test, "john:\$1\$")