Fix lint warnings from invalid escape sequences (#82)

Use raw strings for regex patterns to avoid warnings.

ament_package/templates.py

@@ -111,7 +111,7 @@ def substitute(match):
         if var in environment:
             return environment[var]
         return ''
-    return re.sub('\@[a-zA-Z0-9_]+\@', substitute, template)
+    return re.sub(r'\@[a-zA-Z0-9_]+\@', substitute, template)
 
 
 def _is_platform_specific_extension(filename):