Watch mode broken

[Michał Krassowski]

jlpm watch in jupyterlab hangs and does nothing.

• jupyterlab-server-2.16.3 works
• jupyterlab-server-2.16.4 (yanked) does not work
• jupyterlab-server-2.16.5 does not work

There is not traceback, but if I kill it I see:

$ jlpm watch
yarn run v1.22.19
$ python scripts/watch_dev.py
^CTraceback (most recent call last):
  File "scripts/watch_dev.py", line 10, in <module>
    procs[0].wait()
  File "python3.8/site-packages/jupyterlab_server/process.py", line 146, in wait
    time.sleep(1.0)
KeyboardInterrupt