add missing import pytz in example

The example uses the pytz module:
event.add('dtstart', datetime(2005,4,4,8,0,0,tzinfo=pytz.utc))
But doesn't import it

docs/usage.rst

@@ -263,6 +263,7 @@ Init the calendar::
 
   >>> cal = Calendar()
   >>> from datetime import datetime
+  >>> import pytz
 
 Some properties are required to be compliant::