Expand a recurring event, crashes when EXDATE values are date-only

Describe the bug
When editing an event, Khal will crash if the EXDATE values are date only.

If applicable: Stack Trace

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/khal/ui/__init__.py", line 1494, in start_pane
    loop.run()
    ~~~~~~~~^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/main_loop.py", line 337, in run
    self._run()
    ~~~~~~~~~^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/main_loop.py", line 439, in _run
    self.event_loop.run()
    ~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/select_loop.py", line 182, in run
    self._loop()
    ~~~~~~~~~~^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/select_loop.py", line 229, in _loop
    record.data()
    ~~~~~~~~~~~^^
  File "/usr/lib/python3/dist-packages/urwid/display/_posix_raw_display.py", line 285, in wrapper
    return self.parse_input(event_loop, callback, self.get_available_raw_input())
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/display/_raw_display_base.py", line 488, in parse_input
    callback(decoded_codes, raw_codes)
    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/main_loop.py", line 466, in _update
    self.process_input(keys)
    ~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/event_loop/main_loop.py", line 566, in process_input
    handled_key = self._topmost_widget.keypress(self.screen_size, key)
  File "/usr/lib/python3/dist-packages/urwid/widget/popup.py", line 143, in keypress
    return self._current_widget.keypress(size, key)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/widget/frame.py", line 526, in keypress
    return self.body.keypress((maxcol, remaining), key)
           ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/ui/__init__.py", line 1158, in keypress
    return super().keypress(size, key)
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/ui/base.py", line 121, in keypress
    return super().keypress(size, key)
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/widget/widget.py", line 729, in keypress
    return get_delegate(self).keypress(size, key)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/ui/widgets.py", line 317, in keypress
    key = super().keypress(size, key)
  File "/usr/lib/python3/dist-packages/urwid/widget/columns.py", line 1216, in keypress
    key = w.keypress(size_args[i], key)
  File "/usr/lib/python3/dist-packages/urwid/widget/widget.py", line 729, in keypress
    return get_delegate(self).keypress(size, key)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/urwid/widget/widget.py", line 729, in keypress
    return get_delegate(self).keypress(size, key)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/ui/__init__.py", line 947, in keypress
    self.edit(self.focus_event.event, external_edit=True)  
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
  File "/usr/lib/python3/dist-packages/khal/ui/__init__.py", line 769, in edit
    self.pane.collection.update(new_event)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/khalendar/khalendar.py", line 182, in update
    self._backend.update(event.raw, event.href, event.etag, calendar=event.calendar)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/khalendar/backend.py", line 246, in update
    self._update_impl(vevent, href, calendar)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/khalendar/backend.py", line 367, in _update_impl
    dtstartend = expand_vevent(vevent, href)
  File "/usr/lib/python3/dist-packages/khal/icalendar.py", line 340, in expand
    for date in get_dates(vevent, 'EXDATE') or ():
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/khal/icalendar.py", line 253, in sanitize_datetime
    date = events_tz.localize(date)
  File "/usr/lib/python3/dist-packages/pytz/tzinfo.py", line 320, in localize
    if dt.tzinfo is not None:
       ^^^^^^^^^
AttributeError: 'datetime.date' object has no attribute 'tzinfo'

To Reproduce

• In khal interactive, create a recurring event.
  e.g.:

BEGIN:VCALENDAR
BEGIN:VEVENT
UID:event_rrule_exdate_timezone_date
SUMMARY:Monthly Report
RRULE:FREQ=MONTHLY;BYDAY=+4SA;UNTIL=20301231T060000Z
DTSTART;TZID=Europe/Berlin:20240629T070000
DTEND;TZID=Europe/Berlin:20240629T120000
END:VEVENT
END:VCALENDAR

• Use the external_edit function (meta E) to edit the iCalendar data.
• Set some exception date values (in the EXDATE property) for the recurrence set.
  e.g.:

BEGIN:VCALENDAR
BEGIN:VEVENT
UID:event_rrule_exdate_timezone_date
SUMMARY:Monthly Report
RRULE:FREQ=MONTHLY;BYDAY=+4SA;UNTIL=20301231T060000Z
DTSTART;TZID=Europe/Berlin:20240629T070000
DTEND;TZID=Europe/Berlin:20240629T120000
EXDATE;TZID=Europe/Berlin:20241227,20251227,20261226,20271225,20281223,20291229,20301228
END:VEVENT
END:VCALENDAR

• Save the file and return to Khal.
  • Khal will crash with the above stack trace.

Expected behavior

Because the EXDATE property contains date-only values (not datetime), this still meets the iCalendar specification.

The expected behaviour is to remove any instances from the recurrence set, whose start date matches those date-only values.

OS, version, khal version and how you installed it:

$ cat /etc/debian_version 
13.1

$ dpkg --list khal
[…]
ii  khal           1:0.11.4-1   all          Standards based CLI and terminal calendar program

$ khal --version
khal, version 0.11.4

$ python3 --version
Python 3.13.5

• Your khal config file

[calendars]

[[private]]
path = /home/bignose/.local/share/khal/calendars/private
type = calendar

[locale]
timeformat = %H:%M
dateformat = %Y-%m-%d
longdateformat = %Y-%m-%d
datetimeformat = %Y-%m-%d %H:%M
longdatetimeformat = %Y-%m-%d %H:%M

[default]
default_calendar = private

Additional context

The iCalendar data parses correctly using the Python icalendar library. It crashed Khal only when attempting to render a recurring event from that data.

Start in khal/icalendar.py at sanitize_datetime, get_dates, and expand, then follow the update path through khalendar/backend.py. Reproduce the supplied recurring event with date-only EXDATE values and verify that expansion no longer crashes. Done means recurrence instances whose start dates match the EXDATE values are removed.