diff: fix diffing of subclasses

* Adds regression tests for dict sublasses. (fixes #38)

Co-authored-by: Anders Hovmoller <boxed@killingar.net>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
diff: duplicate isinstance test removal

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>