Support Package fails to download if proxy is in use

After successfully testing the HelloWorld application using briefcase dev I attempted to move on to [tutorial 3](https://docs.beeware.org/en/latest/tutorial/tutorial-3.html) and got the following error:
`Using support package https://briefcase-support.org/python?platform=windows&version=3.6&arch=amd64 Unable to downloading support package; is your computer offline?`

I am behind a Proxy but I configured the git config --global http.proxy and https.proxy using the same proxy configurations I use for pip behind the proxy but it still resulted in the same error. Please advise.

Start by reproducing the failure from tutorial 3 with `briefcase dev` while the machine uses the reported proxy configuration. Trace how Briefcase downloads the support package from the support URL and compare that path with the configured Git and pip proxy settings. Done means support-package downloads work through a proxy and the behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.