Support pyproject.toml for configuration

[rkingsbury]

Thanks for developing this very useful library! I am evaluating it as an alternative to mongomock in a data management library maggma.

Currently pymongo_inmemory.context.conf reads setup.cfg. Is it possible to support pyproject.toml configuration as well? My understanding is that this is now the preferred approach for package configuration.

I can work on a PR if desired. Thanks!