I had to implement this for `vdirsyncer`'s test suite, and this might
save the next person needing it some headaches.

There was a strange issue with dual-stack, so the documentation is updated.

Fixes #61

* httpserver: propagate event handler exceptions to httpserver

* fix import error and add check() method

This has been improved recently to handle assertion errors and to be able to
obtain the unhandled exceptions, so this is now documented.

Also update setup-python to v2.

This file is required to enable type checkers to use the type hints defined
in this library.

Fixes #65.

Add mypy to dev dependencies and wire it in to gitlab actions.
Fix type annotations (currently without "--strict").

Fixes #65.

The reason of this removal is the type hinting syntax introduced in python
3.6, which is added in the previous commit.

Python 3.5 reached end of life in September of 2020, no longer receiving
security fixes so users using it are encouraged to move forward.

This will do git tagging and changelog generation and others, but does not
push anything to anywhere. Also, no pypi upload (yet!).