Use correct syntax for is_file

Author: jaraco

importlib_resources/abc.py

@@ -136,7 +136,7 @@ def resource_path(self, resource):
         raise FileNotFoundError(resource)
 
     def is_resource(self, path):
-        return self.files().joinpath(path).isfile()
+        return self.files().joinpath(path).is_file()
 
     def contents(self):
         return (item.name for item in self.files().iterdir())