Release 3.3.0

cc @woodruffw

Changelog:

• CLI: The sigstore verify command now outputs the inner in-toto statement
  when verifying DSSE envelopes. If verification is successful, the output
  will be the inner in-toto statement. This allows the user to see the
  statement's predicate, which sigstore-python does not verify and should be
  verified by the user.
• CLI: The sigstore attest subcommand has been added. This command is
  similar to cosign attest in that it signs over an artifact and a
  predicate using a DSSE envelope. This commands requires the user to pass
  a path to the file containing the predicate, and the predicate type.
  Currently only the SLSA Provenance v0.2 and v1.0 types are supported.
• CLI: The sigstore verify command now supports verifying digests. This means
  that the user can now pass a digest like sha256:aaaa.... instead of the
  path to an artifact, and sigstore-python will verify it as if it was the
  artifact with that digest.