cli: Display datetimes in local timezone

Previously the start and end datetimes were always printed out in the
timezone that they appear in the calendar entry.

In 7a8d584 duration support was added and an attempt was already made
to display the datetime in the local timezone. Unfortunately in that
specific case the `start.astimezone(start.tzinfo)` is a no-op and does
absolutely nothing. Unlike the name suggests, `astimezone()` adjusts the
date and time data, such that they match the passed tzinfo, but since
that is the same timezone data as before, nothing changes. [0]

In order to properly convert to the user's local timezone, we need to
call the method with no arguments.

With this the timezone is always properly displayed, which makes up for
a much nicer UX for users of the cli.

The test has to be adapted to expect the datetime in the local timezone,
hence we cannot hardcode the entire expected string anymore.

[0] https://docs.python.org/3/library/datetime.html#datetime.datetime.astimezone

CHANGES.rst

@@ -6,7 +6,9 @@ Changelog
 
 Minor changes:
 
-- ...
+- The cli utility now displays start and end datetimes in the user's local timezone.
+  Ref: #561
+  [vimpostor]
 
 Breaking changes:
 

src/icalendar/cli.py

@@ -52,10 +52,10 @@ def view(event):
         end = event.decoded('dtend', default=start)
     duration = event.decoded('duration', default=end - start)
     if isinstance(start, datetime):
-        start = start.astimezone(start.tzinfo)
+        start = start.astimezone()
     start = start.strftime('%c')
     if isinstance(end, datetime):
-        end = end.astimezone(end.tzinfo)
+        end = end.astimezone()
     end = end.strftime('%c')
 
     return f"""    Organizer: {organizer}

src/icalendar/tests/test_cli_tool.py

@@ -1,6 +1,11 @@
 import unittest
 
+from datetime import tzinfo, datetime
 from icalendar import Calendar, cli
+try:
+    import zoneinfo
+except ModuleNotFoundError:
+    from backports import zoneinfo
 
 INPUT = '''
 BEGIN:VCALENDAR
@@ -21,7 +26,7 @@
 ORGANIZER:organizer@test.test
 ATTENDEE:attendee1@example.com
 ATTENDEE:attendee2@test.test
-SUMMARY:Test summury
+SUMMARY:Test summary
 DTSTART;TZID=Europe/Warsaw:20220820T200000
 DTEND;TZID=Europe/Warsaw:20220820T203000
 LOCATION:New Amsterdam, 1010 Test Street
@@ -36,13 +41,22 @@
 END:VCALENDAR
 '''
 
-PROPER_OUTPUT = '''    Organizer: organizer <organizer@test.test>
+def local_datetime(dt):
+    return datetime.strptime(dt, "%Y%m%dT%H%M%S").replace(tzinfo=zoneinfo.ZoneInfo("Europe/Warsaw")).astimezone().strftime('%c')
+
+# datetimes are displayed in the local timezone, so we cannot just hardcode them
+firststart = local_datetime('20220820T103400')
+firstend = local_datetime('20220820T113400')
+secondstart = local_datetime('20220820T200000')
+secondend = local_datetime('20220820T203000')
+
+PROPER_OUTPUT = f"""    Organizer: organizer <organizer@test.test>
     Attendees:
      attendee1 <attendee1@example.com>
      attendee2 <attendee2@test.test>
     Summary    : Test Summary
-    Starts     : Sat Aug 20 10:34:00 2022
-    End        : Sat Aug 20 11:34:00 2022
+    Starts     : {firststart}
+    End        : {firstend}
     Duration   : 1:00:00
     Location   : New Amsterdam, 1000 Sunrise Test Street
     Comment    : Comment
@@ -53,9 +67,9 @@
     Attendees:
      attendee1 <attendee1@example.com>
      attendee2 <attendee2@test.test>
-    Summary    : Test summury
-    Starts     : Sat Aug 20 20:00:00 2022
-    End        : Sat Aug 20 20:30:00 2022
+    Summary    : Test summary
+    Starts     : {secondstart}
+    End        : {secondend}
     Duration   : 0:30:00
     Location   : New Amsterdam, 1010 Test Street
     Comment    : 
@@ -75,7 +89,7 @@
     Description:
      
 
-'''
+"""
 
 class CLIToolTest(unittest.TestCase):
     def test_output_is_proper(self):