This adds type hints for all remaining modules and tests in

djblets.integrations, helping people develop or work with

integrations.
A couple minor issues were caught by fixing up types, but had no impact

on consumers:


IntegrationConfigForm could crash if manually calling

get_key_value() or set_key_value() if creating a new

  configuration.


Unit test setup for IntegrationConfig's manager wasn't correct. We

  were overriding a classmethod with a standard member function.


With this change, djblets.integrations is now fully typed.

Unit tests, mypy, and pyright pass.