bugfix: updated core and gui to new callback method from pytube3 > v …

[chonix]

Fixed the issue where if you had a newer version of pytube3 (9.6.3+) it would break the download and make the app crash.

Haven't tested with a previous pytube version, so I might want to check that later on, as this is a quick and dirty bugfix

Steps to test:

• pip install pytube==9.6.4
• open application
• download file

[AlexPHorta]

Fine. Working here too.