Detect infinite recursion in Entity.to_xml

See merge request !166

Compare_entities diff uses id instead of name where needed

See merge request !165

- Added a visited_entities parameter to save all Entities already treated in to_xml. New calls to to_xml are checked against this to identify potential infinite recursion
- Added test
- Updated changelog

BUG: The diff returned by compare_entities now uses id instead of name as key if either of the compared properties do not have a name

WIP: Modernizing setup.

See merge request !163

Prevent auth_token from being overwritten with None

See merge request !162

Fix mypy errors

See merge request !164