v1.8.0 — Fix provenance support

• Provenance support belatedly updated to match a change to PEP 740:
  • DistributionPackage.provenance_sha256 is now deprecated and is always None
  • DistributionPackage.provenance_url is now determined correctly and is None when no provenance file is declared
  • PyPISimple.get_provenance() no longer verifies the provenance's digest, and the verify argument is now deprecated