[Python] Improve model equality implementation (#4663)

* [Python] Improve model equality implementation

The model equality implementation template blindly tried accessing the
`__dict__` attribute of the variable to compare equality against. This
attribute does not exist for a lot of Python built-in types (`None`,
`str` etc.) and the equality check would simply crash.

This adds a simple guard to only continue with equality check if the
variable to compare is an instance of the model being compared against.

* Remove wrong auto-update of Python requirements

modules/swagger-codegen/src/main/resources/python/model.mustache

@@ -172,6 +172,9 @@ class {{classname}}(object):
         """
         Returns true if both objects are equal
         """
+        if not isinstance(other, {{classname}}):
+            return False
+
         return self.__dict__ == other.__dict__
 
     def __ne__(self, other):