Description of problem:
I installed trytond-calendar, but not pywebdav and the trytond service was not starting. After installing pywebdav the service started.

Version-Release number of selected component (if applicable):
trytond-calendar-1.8.1-1.fc15.noarch

How reproducible:
Every time

Steps to Reproduce:
1. Install trytond-calendar, but not pywebdav
  
Actual results:
The trytond service fails to start.

Expected results:
The trytond service should start.

Additional info:
While debugging, I managed to get this:
Traceback (most recent call last):
  File "/usr/bin/trytond", line 30, in <module>
    trytond.server.TrytonServer().run()
  File "/usr/lib/python2.7/site-packages/trytond/server.py", line 99, in run
    Pool.start()
  File "/usr/lib/python2.7/site-packages/trytond/pool.py", line 75, in start
    register_classes(reload_p=reload_p)
  File "/usr/lib/python2.7/site-packages/trytond/modules/__init__.py", line 378, in register_classes
    pathname, description)
  File "/usr/lib/python2.7/site-packages/trytond/modules/calendar/__init__.py", line 4, in <module>
    import caldav
  File "/usr/lib/python2.7/site-packages/trytond/modules/calendar/caldav.py", line 7, in <module>
    from DAV import propfind
ImportError: No module named DAV