Fix anomalous backslashes in strings

Python 3.8 emits a SyntaxWarning for a string with an anomalous
backslash, e.g.:

  <unknown>:127: SyntaxWarning: invalid escape sequence \s

Fix by adding the missing 'r' prefix.

Author: pkolbus

src/license_expression/__init__.py

@@ -128,7 +128,7 @@ class ExpressionParseError(ParseError, ExpressionError):
 # mapping of lowercase operator strings to an operator object
 OPERATORS = {'and': KW_AND, 'or': KW_OR, 'with': KW_WITH}
 
-_simple_tokenizer = re.compile('''
+_simple_tokenizer = re.compile(r'''
     (?P<symop>[^\s\(\)]+)
      |
     (?P<space>\s+)

src/license_expression/_pyahocorasick.py

@@ -608,7 +608,7 @@ def overlap(self, other):
 
 
 # tokenize to separate text from parens
-_tokenizer = re.compile('''
+_tokenizer = re.compile(r'''
     (?P<text>[^\s\(\)]+)
      |
     (?P<space>\s+)