Type Hinting

[AndKaminer]

This goes well with #54, but since technically type hinting isn't part of PEP 8, I pulled it out here. Is type hinting something we want to do? We can check it with something like mypy, which can be automated just like unit testing and flake8. Thoughts?

[MarcCote]

Type hinting is always great. I like the idea of running mypy but I wouldn't make it as a mandatory check for now.

[pangwangshu]

Opened #89 to complete this — extends the type hints already started on ScienceWorldEnv's snake_case API to the remaining untyped surface (utils.py, the deprecated camelCase wrapper methods, BufferedHistorySaver, etc.), plus two small bugfixes mypy caught along the way.

Per @MarcCote's comment, added mypy as an opt-in tox env (tox -e mypy) rather than a mandatory CI check for now.