added list(d.keys()) for Python3 compatibility
added list(d.keys()) for Python3 compatibility

Vicaris/ModPro,Zulko/moviepy,mcanthony/moviepy,benroeder/moviepy,carlodri/moviepy,bertyhell/moviepy,LunarLanding/moviepy,misterbisson/moviepy,ssteo/moviepy,kerstin/moviepy,TheNathanBlack/moviepy,deniszgonjanin/moviepy