Add start_ws method to pygls' LanguageClient

[alcarney]

Similar to #501, this adds WebSockets support to pygls' LanguageClient.

• In CI, the end-to-end tests are now also run over WebSockets
• This has highlighted another instance of Unable to send messages from threaded handlers over TCP on Windows #502, this time affecting all platforms I believe.
  The corresponding test has been skipped, until we can find a solution.
• The minimum version of websockets has been bumped to 13.0

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

Automated linters

You can run the lints that are run on CI locally with:

poetry install --all-extras --with dev
poetry run poe lint

[alcarney]

Thanks!