Add support for Raspberry Pi

briefcase create fails >
Unable to download https://briefcase-support.org/python?platform=linux&version=3.7&arch=armv7l; is the URL correct?
followed tutorial exactly >https://docs.beeware.org/en/latest/tutorial/tutorial-3.html
Environment:**
- Rpi OS:
- python 3.5:
- Software versions:
- Briefcase:
- Toga:

Start by reproducing `briefcase create` while following tutorial-3.html and inspect the failed support URL for the reported ARM architecture. The issue does not name implementation files or tests; done means Briefcase can create the tutorial application successfully on the reported Raspberry Pi environment.

Written by the indexing model from the issue text.