Adding mypy typing

[william-stearns]

Initial attempt to add python type hints to the majority of the (non pb2) python files in this repo. There are a few notes about things that may need attention, but it's largely focused on type hints.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

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[william-stearns]

I don't know how you'd like to see PRs generated. Please let me know if I need to do anything in another way to fit with your workflow.

[holdenweb]

It appears the only thing that is currently holding up the PR (besides a committer with time to review it) is the non-existence of the BLEClient entity reference in ble_interface.py, line 121.

The annotation says you are trying to return an object of a non-existent class - just click on the "Details" links under "Some checks were not successful".

[william-stearns]

Thank you so much, @holdenweb ! I hadn't realized the ball was back in my court.
I've removed the references to BLEClient typing, with the hope this is enough to get it going.