Trio-websocket

Fixed
- fixed a race condition where, just after a local-initiated close, the
`closed` attribute would be `None`, and `send_message()` would be silently
ignored (wsproto < 0.2.0) or leak a `LocalProtocolError` (wsproto >= 0.2.0)
rather than raise `ConnectionClosed`
([158](https://github.com/python-trio/trio-websocket/issues/158))

Fixed
- `send_message()` is changed to raise `ConnectionClosed` when a close
handshake is in progress. Previously, it would silently ignore
the call, which was an oversight, given that `ConnectionClosed` is
defined to cover connections "closed or in the process of closing".
Notably, this fixes `send_message()` leaking a wsproto `LocalProtocolError`
with wsproto >= 1.2.0.
([175](https://github.com/python-trio/trio-websocket/issues/175))

Released as a minor version increment, since code calling `send_message()`
is expected to handle `ConnectionClosed` anyway.

Fixed
- avoid MultiError warnings with trio >= 0.22
Changed
- drop support for Python 3.5, 3.6

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.)