relationship validation must allow NONE and NOASSERTION

[meretp]

The spec explicitly allows the keywords NOASSERTION or NONE on the right hand side of a relationship (see here). When trying to parse the example file SPDXJSONExample-v2.3.spdx.json the validation returns the following messages:

ValidationMessage(validation_message='spdx_id must only contain letters, numbers, "." and "-" and must begin with "SPDXRef-", but is: NOASSERTION', context=ValidationContext(spdx_id=None, parent_id=None, element_type=<SpdxElementType.RELATIONSHIP: 13>, full_element=Relationship(spdx_element_id='SPDXRef-CommonsLangSrc', relationship_type=<RelationshipType.GENERATED_FROM: 25>, related_spdx_element_id='NOASSERTION', comment=None))), 
ValidationMessage(validation_message='did not find the referenced spdx_id NOASSERTION in the SPDX document', context=ValidationContext(spdx_id=None, parent_id=None, element_type=<SpdxElementType.RELATIONSHIP: 13>, full_element=Relationship(spdx_element_id='SPDXRef-CommonsLangSrc', relationship_type=<RelationshipType.GENERATED_FROM: 25>, related_spdx_element_id='NOASSERTION', comment=None)))

[armintaenzertng]

fixed by #390