xsd:date literals with negative years raise ISO8601Error due to isodate limitation

[ch-sander]

when parsing RDF data containing valid xsd:date literals with years before 0000 (e.g. "-0012-11-29"^^xsd:date), rdflib raises an exception:

isodate.isoerror.ISO8601Error: Unrecognised ISO 8601 date format: '-0012-11-29'

According to ISO 8601 and XML Schema, negative years are allowed and represent years BCE using a proleptic Gregorian calendar. The literal above correctly represents 13 BCE.

The error occurs because rdflib relies on the isodate package for parsing, which does not support negative years.

Any workaround or fix?