test: "CCYYMMDD" is not a valid XSD date

XSD-date cannot be written in the form "CCYYMMDD", this patch adds a
test to ensure it.

linkml-runtime has been relying on Python `fromisoformat` [1] format to
parse and, implicitly validate, XSD-date. But from Python 3.11 upwards
`fromisoformat` also accepts "CCYYMMDD" as a valid date, breaking
XSD-date compatibility.

This is a regression test to avoid this issue in the future.

[1]: https://docs.python.org/3/library/datetime.html#datetime.date.fromisoformat

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Author: Silvanoc

tests/test_utils/test_metamodelcore.py

@@ -162,6 +162,8 @@ def test_date(self):
         self.assertEqual('2019-07-06', v.isoformat())                           # A string
         self.assertEqual('2019-07-06', XSDDate(XSDDate(v)))                     # An existing date
         strict()
+        with self.assertRaises(ValueError):
+            XSDDate('20190706')
         with self.assertRaises(ValueError):
             XSDDate('Jan 12, 2019')
         with self.assertRaises(ValueError):