test_create_ical is flaky

[Lukas Märdian]

We're seeing some flakyness with the `test_create_ical` test case on Ubuntu's heavily loaded autopkgtest infrastructure. It seems like there's a timing issue, where test data can be generated at different points in time and the test fails when the data is not generated in the same second, see below:

299s E AssertionError: b'BEG[45 chars]**163454Z**\nDTSTART:20321010T101010Z\nDURATION:PT[96 chars]:2.0' != b'BEG[45 chars]**163455Z**\nDTSTART:20321010T101010Z\nDURATION:PT[96 chars]:2.0'

=> See "163454Z" vs "163455Z"

Notes:
There is also a deprecation warning when using python3.12, I don't know if that might have an impact:
```
299s /usr/lib/python3/dist-packages/dateutil/tz/tz.py:37: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
299s EPOCH = datetime.datetime.utcfromtimestamp(0)
```

Full logs (https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/armhf/p/python-caldav/20240206_163515_8815a@/log.gz):
```
294s I: pybuild base:305: cd /tmp/autopkgtest.l1XavR/autopkgtest_tmp/build; python3.12 -m pytest tests
298s ============================= test session starts ==============================
298s platform linux -- Python 3.12.1, pytest-7.4.4, pluggy-1.3.0
298s rootdir: /tmp/autopkgtest.l1XavR/autopkgtest_tmp/build
298s collected 107 items
298s
298s tests/test_caldav.py sssssssssssssssssssssssssssssssssssssssssssssssssss [ 47%]
298s ssssssssssssssssssssss [ 68%]
298s tests/test_caldav_unit.py ...s...................... [ 92%]
298s tests/test_cdav.py ...... [ 98%]
298s tests/test_utils.py . [ 99%]
299s tests/test_vcal.py F [100%]
299s
299s =================================== FAILURES ===================================
299s __________________________ TestVcal.test_create_ical ___________________________
299s
299s self = <tests.test_vcal.TestVcal testMethod=test_create_ical>
299s
299s def test_create_ical(self):
299s def create_and_validate(**args):
299s return self.verifyICal(create_ical(**args))
299s
299s ## First, a fully valid ical_fragment should go through as is
299s self.assertSameICal(create_and_validate(ical_fragment=ev), ev)
299s
299s ## One may add stuff to a fully valid ical_fragment
299s self.assertSameICal(
299s create_and_validate(ical_fragment=ev, priority=3), ev + "\nPRIORITY:3\n"
299s )
299s
299s ## binary string or unicode string ... shouldn't matter
299s self.assertSameICal(
299s create_and_validate(ical_fragment=ev.encode("utf-8"), priority=3),
299s ev + "\nPRIORITY:3\n",
299s )
299s
299s ## The returned ical_fragment should always contain BEGIN:VCALENDAR and END:VCALENDAR
299s ical_fragment = ev.replace("BEGIN:VCALENDAR", "").replace("END:VCALENDAR", "")
299s self.assertSameICal(create_and_validate(ical_fragment=ical_fragment), ev)
299s
299s ## Create something with a dtstart and verify that we get it back in the ical
299s some_ical0 = create_and_validate(
299s summary="gobledok",
299s dtstart=datetime(2032, 10, 10, 10, 10, 10, tzinfo=utc),
299s duration=timedelta(hours=5),
299s )
299s some_ical1 = create_and_validate(
299s summary=b"gobledok",
299s dtstart=datetime(2032, 10, 10, 10, 10, 10, tzinfo=utc),
299s duration=timedelta(hours=5),
299s )
299s assert re.search(b"DTSTART(;VALUE=DATE-TIME)?:20321010T101010Z", some_ical0)
299s > self.assertSameICal(some_ical0, some_ical1, ignore_uid=True)
299s
299s tests/test_vcal.py:103:
299s _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
299s tests/test_vcal.py:48: in assertSameICal
299s self.assertEqual(normalize(ical1, ignore_uid), normalize(ical2, ignore_uid))
299s E AssertionError: b'BEG[45 chars]163454Z\nDTSTART:20321010T101010Z\nDURATION:PT[96 chars]:2.0' != b'BEG[45 chars]163455Z\nDTSTART:20321010T101010Z\nDURATION:PT[96 chars]:2.0'
299s =============================== warnings summary ===============================
299s ../../../../usr/lib/python3/dist-packages/dateutil/tz/tz.py:37
299s /usr/lib/python3/dist-packages/dateutil/tz/tz.py:37: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
299s EPOCH = datetime.datetime.utcfromtimestamp(0)
299s
299s -- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
299s =========================== short test summary info ============================
299s FAILED tests/test_vcal.py::TestVcal::test_create_ical - AssertionError: b'BEG...
299s ============= 1 failed, 32 passed, 74 skipped, 1 warning in 4.28s ==============
299s E: pybuild pybuild:391: test: plugin distutils failed with: exit code=1: cd /tmp/autopkgtest.l1XavR/autopkgtest_tmp/build; python3.12 -m pytest tests
```

[Lukas Märdian (slyon)]

Forwarded upstream: https://github.com/python-caldav/caldav/issues/380