Remove frozendict dependency since it appears to be a dead project

[obi1kenobi]

Getting a deprecation warning on frozendict:

frozendict/__init__.py:16: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working
    class frozendict(collections.Mapping):

https://github.com/slezica/python-frozendict appears to have had no activity in years, and there are several easy-to-merge PRs that have had no activity or comments from the maintainer in many months.

I think the package is unmaintained and we should remove our dependency on it.