lms problem after deploying XBlock into devstack

[chemseddine zaafrane]

i recently installed an XBlock named blocktest into Devstack but when i tried to run the LMS i got this error

File "/edx/app/edxapp/venvs/edxapp/bin/paver", line 11, in <module>
    sys.exit(main())
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/paver/tasks.py", line 878, in main
    _launch_pavement(args)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/paver/tasks.py", line 846, in _launch_pavement
    exec(compile(source, environment.pavement_file, 'exec'), mod.__dict__)
  File "pavement.py", line 14, in <module>
    from pavelib import *
  File "pavelib/__init__.py", line 4, in <module>
    from . import assets, servers, docs, prereqs, quality, tests, js_test, i18n, bok_choy, acceptance_test
  File "pavelib/assets.py", line 15, in <module>
    from watchdog.observers.polling import PollingObserver
ImportError: cannot import name PollingObserver

i couldn't find a proper answer to fix this so i decided to uninstall the XBlock, i checked with pip freeze and it was with the name blocktest-xblock then i uninstalled it withg the command pip uninstall blocktest-xblock. When i tried to run the lms after that, the previous error still pops out