Fixed #19746 -- Allow deserialization of pk-less data
Merge pull request #717 from slurms/ticket_19746

Fixed #19746 -- Allow deserialization of pk-less data
Test case and docs for custom context data in feeds

Thanks Paul Winkler for the initial patch. (Ref #18112).
Added a note about Feed.get_context_data to the 1.6 release notes.
Fixed creation of html docs on python 3.

Thanks to Alan Lu for the report and the patch.