diff: richer docstring on new `ignore` parameter

* Enhances docstring of `diff()` in order to document exact version
  number when the `ignore` parameter was added.  (see #34) (see PR #35)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
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>