[FIX] calendar: support non-ascii chars in date/time format

The calendar module generates string values with
a date/time formatted according to the user
language. Those formats may contain non-ascii
characters and are read as unicode strings,
but fed to str{p,t}time, which only accepts
byte strings (in Python 2).

This would cause an exception when loading calendar
notifications for a user using e.g. Chinese with
some CJK unicode chars in the date/time format.