Adds type hints throughout Hatchet and integrate checks with mypy

[ilumsden]

As several of us (particularly myself and @pearce8) have mentioned on numerous occasions, Python's duck typing system can be a pain to deal with.

To help with this, this PR adds extensive type hinting throughout Hatchet to help with understanding what is valid input/output for functions. Additionally, this PR adds support for mypy: the most widely used static type checker for Python. mypy makes use of the type hints added in this PR to determine whether code is written correctly, similar to how a compiler works for languages like C and C++.

Besides type checking, this PR corrects numerous bugs that mypy revealed, ranging from minor typing issues due to reusing variables too much to functions not returning when they are supposed to (this one may actually be part of the cause of the squash issues you were having @slabasan). One important set of things that mypy pointed out is logic for Python 2. Since we are now officially supporting Python 3.5+, this logic is no longer needed, so I have removed any such logic that mypy detected.

Finally, this PR adds mypy-based static type checking to our CI. Like flake8 and black, this will only run for Python 3.9.

Edit:

I got fed up with Black giving different formatting with different versions, so I added a devcontainer to Hatchet. This lets developers who user VSCode work in a Docker-based environment with the exact right versions of tools like black, flake8, and mypy.

[ilumsden]

Also, as a small add-on to this PR, I've updated the .gitignore using the template from GitHub: https://github.com/github/gitignore/blob/main/Python.gitignore