Switch authentication from query parameters to HTTP basic authentication

[fingolfin]

For details, see https://developer.github.com/changes/2019-11-05-deprecated-passwords-and-authorizations-api/

This is somewhat urgent, as GitHub will remove the old mechanism on July 1st, 2020. So, what needs to be done is:

• implement support for new auth methods
• test it with a couple releases
• then notify all people using this script about the update

Not quite sure how to reach "all" users, but I'll at least announce it on the GAP Slack and the main GAP mailing list (perhaps also the forum?). Perhaps also the GAP Forum?

[fingolfin]

First part done in f8ce1aa