- update to 0.13.0:
* Clarify that the Headers class is a Sequence and inherit from the
collections Sequence abstract base class to also indicate this (and
gain the mixin methods).
* Switch event classes to dataclasses for easier typing and slightly
improved performance.
* Shorten traceback of protocol errors for easier readability
* Add typing including a PEP 561 marker for usage by type checkers
* Expand the allowed status codes to [0, 999] from [0, 600]
* Ensure request method is a valid token