Traceback (most recent call last):
  File "/usr/bin/calibre", line 20, in <module>
    sys.exit(calibre())
  File "/usr/lib/calibre/calibre/gui_launch.py", line 72, in calibre
    from calibre.gui2.main import main
  File "/usr/lib/calibre/calibre/gui2/main.py", line 28, in <module>
    from calibre.utils.ipc import RC, gui_socket_address
  File "/usr/lib/calibre/calibre/utils/ipc/__init__.py", line 15, in <module>
    from polyglot.functools import lru_cache
  File "/usr/lib/calibre/polyglot/functools.py", line 11, in <module>
    from backports.functools_lru_cache import lru_cache
ImportError: No module named functools_lru_cache