Verify upstream sources with GPG
    
    This is now a recommended thing to do:
    https://docs.fedoraproject.org/en-US/packaging-guidelines/#_source_file_verification
    
    Regardless if it adds actual security, it should prevent problems like this one:
    https://mail.python.org/archives/list/python-dev@python.org/message/OYNQS2BZYABXACBRHBHV4RCEPQU5R6EP/