docs: def strip_quotes(self, content):
          """
          Unquote given rule.
  
          Args:
              content (str): An import rule.
  
          Raises:
              InvalidImportRule: Raise exception if the rule is badly quoted
              (not started or not ended quotes).
  
          Returns:
              string: The given rule unquoted.
          """
          error_msg = "Following rule is badly quoted: {}"
          if (content.startswith('"') and content.endswith('"')) or \
             (content.startswith("'") and content.endswith("'")):
              return content[1:-1]
          # Quote starting but not ended
          elif (content.startswith('"') and not content.endswith('"')) or \
               (content.startswith("'") and not content.endswith("'")):
              raise InvalidImportRule(error_msg.format(content))
          # Quote ending but not started
          elif (not content.startswith('"') and content.endswith('"')) or \
               (not content.startswith("'") and content.endswith("'")):
              raise InvalidImportRule(error_msg.format(content))
  
          return content