Add pySBOL3 unit tests

[tcmitchell]

Add unit tests that use pySBOL3 to round trip and validate the files in the SBOL3 directory.

[tcmitchell]

This is waiting for pySBOL3 1.0b9 which implements the refersTo change that has already been incorporated into the SBOLTestSuite/SBOL3 files.