Make apps for other platforms like android with custom python libraries

[arian81]

Is your feature request related to a problem? Please describe.
I put a lot of time to develop an app but just when I compiled and run it on android the app would crash because it doesn't have access to custom libraries installed by pip.

Describe the solution you'd like
A simple way to add custom libraries as requirements to be compiled alongside the main app so it would work.

Additional context

[freakboy3742]

Thanks for the report; however, this isn't a Toga problem - it's a Briefcase problem. See beeware/briefcase#471 for details.