Add type information for mypy support

[nth10sd]

Is your feature request related to a problem? Please describe.
Python 3.6 is now EOL, so with the minimum supported Python version at 3.7, we can possibly use from __future__ import annotations and start adding type hints for mypy support.

Describe the solution you'd like
Add type information.

Describe alternatives you've considered
There is currently no type information.

Additional context
None.

[freakboy3742]

Type annotations have been legal syntax since Python 3.0, so the 3.6 lifespan isn't a consideration.

Regardless, type information isn't an inherent and universal good. It can be helpful as an enhancement to documentation, but it can also impede readability in some circumstances.

There is a broader conversation that needs to be had here, in the context of the entire project.

I'm closing this for now, on the basis that having the ticket open won't progress this feature at all. If you want to advocate for adding annotations, I suggest starting a discussion on the BeeWare Discord.