[icalendar] Update to 6.0.0 (#12706)

stubs/icalendar/@tests/stubtest_allowlist.txt

@@ -1,9 +1,23 @@
 # Command line app
-icalendar\.cli
+icalendar.cli
 
 # Tests
 icalendar\.tests(\..*)?
 
 # Methods that use `int` to mean `bool`.
-icalendar\.cal\.Component\.get_inline
-icalendar\.cal\.Component\.set_inline
+icalendar.Component.get_inline
+icalendar.Component.set_inline
+icalendar.cal.Component.get_inline
+icalendar.cal.Component.set_inline
+
+# Runtime __all__ has extra items that don't exist.
+icalendar.__all__
+icalendar.vTypesFactory
+icalendar.parser.__all__
+icalendar.parser.tzid_from_dt
+# Runtime __all__ is missing a comma, leading to incorrect items.
+icalendar.parser_tools.__all__
+icalendar.parser_tools.ICAL_TYPEdata_encode
+
+# Stubtest gets confused by multiple inheritance.
+icalendar.prop.vSkip.__new__