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

Base: 78.07% // Head: 78.05% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (7e033d5) compared to base (96a4f6a).
Patch coverage: 50.00% of modified lines in pull request are covered.

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     

Impacted Files	Coverage Δ
...ter_server/services/kernels/connection/channels.py	47.48% <0.00%> (ø)
jupyter_server/base/zmqhandlers.py	100.00% <100.00%> (ø)
jupyter_server/serverapp.py	76.59% <0.00%> (-0.18%)	⬇️

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[blink1073]

Thanks!