Trio-websocket

Fixed
- the server will now correctly close the TCP stream on a CloseConnection event
([115](https://github.com/python-trio/trio-websocket/issues/115))

Fixed
- fix client open_websocket_url() when the URL path component is empty
([148](https://github.com/python-trio/trio-websocket/issues/148))

> **_NOTE:_** `wsaccel`, which was important for good performance of
>`wsproto <= 0.14`, has been dropped as a trio-websocket requirement. So
> ensure that your app either upgrades to `wsproto >= 0.15` or explicitly
> requires `wsaccel`.

Changed
- allow dependency on recent `wsproto` versions
- eliminate `yarl`, `ipaddress`, and `wsaccel` dependencies
Fixed
- avoid contributing to dropped exceptions during finalization.
(See Trio issue https://github.com/python-trio/trio/issues/1559 for background.)

Fixed
- reader task no longer raises unhandled exception on ClosedResourceError
([134](https://github.com/python-trio/trio-websocket/issues/134))
- minor issues in example code, documentation, and type-hinting

...