Python 3 is a new version of the language that is incompatible with the 2.x line of releases. The language is mostly the same, but many details, especially how built-in objects like dictionaries and strings work, have changed considerably, and a lot of deprecated features have finally been removed.
This is a companion discussion topic for the original entry at https://copr.fedorainfracloud.org/coprs/bsavelev/python3/