Tornado WebSocketHandler fixup

[davidbrochart]

Fixes #1081.

[davidbrochart]

Although jupyterlab-lsp should not import Tornado's WebSocketHandler from jupyter_server.base.zmqhandlers, but from tornado.websocket directly.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.05%. Comparing base (96a4f6a) to head (7e033d5).
Report is 285 commits behind head on main.

Files	Patch %	Lines
...ter_server/services/kernels/connection/channels.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1083      +/-   ##
==========================================
- Coverage   78.07%   78.05%   -0.03%     
==========================================
  Files          68       68              
  Lines        8298     8299       +1     
  Branches     1652     1652              
==========================================
- Hits         6479     6478       -1     
- Misses       1420     1421       +1     
- Partials      399      400       +1     

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[blink1073]

Thanks!