feat: improve XSD-date validation

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.

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

Author: Silvanoc

linkml_runtime/utils/metamodelcore.py

@@ -278,6 +278,8 @@ def is_valid(cls, value: Union[str, datetime.date, Literal]) -> bool:
             value = value.value
         if isinstance(value, datetime.date):
             value = value.isoformat()
+        if not re.match(r'^\d{4}-\d{2}-\d{2}$', value):
+            return False
         try:
             datetime.date.fromisoformat(str(value))
         except ValueError: