Fix for async types (#515)

### Changes

Statically checking attr's is failing for `AsyncAuth0` because the
attributes are added dynamically.

I've disabled that type-checking rule because - type support for this
SDK is only very basic at the moment. But we will improve it in the
future if/when we can get structured API data (like OpenAPI 3) from the
Auth0 Management API.

### References

fixes #514

mypy.ini

@@ -6,7 +6,7 @@ ignore_errors = True
 
 [mypy-auth0.management.*]
 ignore_errors = False
-disable_error_code=var-annotated
+disable_error_code=var-annotated, attr-defined
 
 [mypy-auth0.rest_async]
 disable_error_code=override