Log Message:
py-wsproto: updated to 0.15.0

0.15.0:
* Drop support for Python 2. Please pin to ~= 0.14.0 if you support
  Python 2.
* Drop support for Python 3.5, meaning the minimum supported version
  is Python 3.6.1.
* Switch events to be dataclass based, otherwise the API is
  consistent.
* Add type hints throughout and support PEP 561 via a py.typed
  file. This should allow projects that use wsproto to type check their
  usage of wsproto.
* Bugfix prevent the test folder being installed as a package called
  test.
* Explicitly require Host header in handshake.
* Drop wsaccel support and utilise the aiohttp/@willmcgugan masking
  method. wsaccel is unmaintained and this new maksing method is
  almost as quick.