Update appcast script for python3
Latest objective-git
Latest objective-git: API changes

- [GTRepo lookupObjectByRevParse:error:] -> - [GTRepo lookUpObjectByRevParse:error:]
Sidestep error due to wrong class in the submodules collection

Remove PBGitSubmodule for now; just use GTSubmodule.
Merge remote-tracking branch 'refs/remotes/origin/master'