TRAPI 1.5 + some Biolink 4.2 validation, with added back support of Python 3.9

• Reasoner-validator is TRAPI 1.5 aware but nothing more than basic schema validation provided; unit tests which are incompletely refactored are skipped.
• Biolink Model 4.2.0 validation starting to be added : validate presence and values of "knowledge_level" and "agent_type", with compliant values, on all knowledge graph edges (removed previous ATTRIBUTE_TYPE_ID_INCLUSIONS for these fields, since no longer surprising to see them...)
• Update BiolinkValidator.get_result() to properly return Tuple[str, MESSAGES_BY_TARGET]
• Cleaned up some technical debt
• Some environments (i.e. ARAX) still work with Python 3.9, so add back support for this