V2 Claims in Python SDK

[gpeacock]

[tmathern]

v2 claims will become the default signed claims once #140 is merged (the claims version can also be made explicit by setting the "claim_version" property in the manifest).
Note that v1 claims can still be used by setting the claim_version on the manifest ("claim_version": 1).

[tmathern]

Fix published in https://pypi.org/project/c2pa-python/0.13.0/.
v2 claims are now the default.