fix: catch ValueError in _to_utc_date_string for year-1 datetimes

astimezone(utc_tz) on a naive datetime near year 1 can raise ValueError
("year must be in 1..9999, not 0") when the local timezone offset shifts
the date before year 1, not only OverflowError/OSError as previously assumed.

Fixes tests TestErrorUnitTestBot::test_to_utc_date_string_with_exception
and TestRegressionUnitTestBot::test_to_utc_date_string_min broken by e9a053a.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

caldav/elements/cdav.py

@@ -20,7 +20,7 @@ def _to_utc_date_string(ts):
             ## in python 3.6 and higher, ts.astimezone() will assume a
             ## naive timestamp is localtime (and so do we)
             ts = ts.astimezone(utc_tz)
-        except (OverflowError, OSError):
+        except (OverflowError, OSError, ValueError):
             ## native time stamp and the current python version is
             ## not able to treat it as localtime.
             import tzlocal