Add pypy3 to tox.ini.
Remove workarounds for ancient versions of libcurl.
Remove support for the old get_error_html function.
Increase the test timeout on travis.
Update tests after the removal of get_error_html.
Edit comments to discourage creation of new OutputTransforms.
Drop support for the draft76 version of WebSockets.

Browsers that only support draft76 are now less common than those
that do not support websockets at all, so applications should
use their non-websocket workarounds for these browsers.
Update docs for deprecations and removals.