Relax type checking on ExtensionApp.serverapp

[minrk]

Some extensions (e.g. JupyterLab) apparently may be loaded hooked up to a legacy NotebookApp, which is not a ServerApp.

Hopefully my last fix trying to finish #730

Launching jupyter notebook with jupyterlab installed fails to load with:

    traitlets.traitlets.TraitError: The 'serverapp' trait of a LabApp instance expected a ServerApp, not the NotebookApp at '0x1035b51c0'.

[blink1073]

Thanks!

[codecov-commenter]

Codecov Report

Merging #739 (0c708f2) into main (a97edaa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #739   +/-   ##
=======================================
  Coverage   70.64%   70.64%           
=======================================
  Files          62       62           
  Lines        7607     7607           
  Branches     1215     1215           
=======================================
  Hits         5374     5374           
  Misses       1865     1865           
  Partials      368      368           

Impacted Files	Coverage Δ
jupyter_server/extension/application.py	73.47% <100.00%> (ø)

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[blink1073]

Holy moly, we can now just re-run failed jobs only!

[minrk]

Awesome! That was the very first feature request I +1'd when we switched to GHA.

[kathoef]

Wow, thank you a lot for fixing (and releasing) this so quickly!