Initial type annotations and mypy setup

[jodal]

This adds a mypy lint task in tox and CI and adds typing to a number of modules.

One use-before-definition bug in an exception handler has already been caught thanks to mypy. Bonus points for finding it in the diff ;-)

Modules that have type annotations are, until we're fully typed, explicitly added to setup.cfg. This way, we can gradually type more and more of Mopidy, similarly to how we gradually made more and more of the test suite pass on Python 3 in late 2019.

I've made extensive use of from __future__ import annotations in combination with keeping typing-only imports inside if TYPE_CHECKING. This ensures that the runtime performance hit from the type annotations is minimal.

[adamcik]

Looking forward to being able to replace my validation code with this + some decorator :-)

[kingosticks]

This is my first exposure to Python typing - I might have made dumb suggestions...

[kingosticks]

Running tox on the latest develop branch on my (hopefully now updated) development environment fails:

mypy run-test: commands[0] | python -m mypy mopidy
mopidy/audio/scan.py:22: error: First argument to namedtuple() should be '_Result', not 'Result'
Found 1 error in 1 file (checked 67 source files)

Do I have a bad/wrong version of mypy?

[kingosticks]

Ah, I have:
mypy==0.800
vs when you merged:
mypy==0.790

Just my luck! They changed it.