Docs: Fix reference target warnings related to flask_restful (#6034)

The REST API dependency `flask_restful` released `0.3.10` recently which
caused the RTD build to start warning about the `Api` and `Resource`
classes no longer being findable. The classes are still importable and
looking at the changes with `0.3.9` it is not clear that the imports
should have changed, so for now these are added to the nitpick ignore
list.

docs/source/nitpick-exceptions

@@ -160,6 +160,8 @@ py:class html.parser.HTMLParser
 
 py:class disk_objectstore.container.Container
 
+py:class flask_restful.Api
+py:class flask_restful.Resource
 py:class flask.app.Flask
 py:class Flask