Fix RuboCop grouped expression warning in Python lexer (rouge-ruby#1513)

The use of a ternary in code added to the Python lexer as part of the
rouge-ruby#1508 pull request results in a RuboCop warning concerning parentheses
and grouped expressions. This commit fixes that warning.

lib/rouge/lexers/python.rb

@@ -210,7 +210,7 @@ def current_string
           | [0-7]{1,3}
           )
         )x do
-          token (current_string.type?("r") ? Str : Str::Escape)
+          current_string.type?("r") ? token(Str) : token(Str::Escape)
           pop!
         end