fix(checker): use raw string for all regex

Avoid DeprecationWarning ("invalid escape sequence").

src/pydocstyle/checker.py

@@ -977,15 +977,15 @@ def is_ascii(string):
 
 def leading_space(string):
     """Return any leading space from `string`."""
-    return re('\s*').match(string).group()
+    return re(r'\s*').match(string).group()
 
 
 def get_leading_words(line):
     """Return any leading set of words from `line`.
 
     For example, if `line` is "  Hello world!!!", returns "Hello world".
     """
-    result = re("[\w ]+").match(line.strip())
+    result = re(r"[\w ]+").match(line.strip())
     if result is not None:
         return result.group()