Updated requirements

Gradio (4.0.2) throws multiple errors. Firstly, the gr.Image takes list of sources instead of source. 'source' parameter is not recognized by this version of Gradio. Secondly, the gr.outputs is also not recognized by this version. The version '3.5.0' does not give any errors. All in all, 2 changes: 1. "source" to "sources" 2. gradio==4.0.2 to gradio==3.5.0