This adds PEP 561 style type hints for Pybricks v2 (just EV3).

This is the v2 branch which only supports EV3. Removing the extra APIs
will prevent them from showing up when this library is used for code
completion.

As per standard Python socketserver, there is `server_close()` rather than `close()`.

Fixes #51

The Microsoft Python extension for VS code doesn't include imports in
type hints, so the Motor type wasn't working properly. We can fix this
by wrapping the import type in a new, empty class definition.

The DCMotor class was also missing from ev3devices so this is fixed as well.