As a consequence:
• Bump minimum supported Python version to 3.7.
• Move Vagrant environment to Debian 10, which has Python 3.7.
• Move CI frontend tests to Debian 10.
• Move production build test to Debian 10.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
CircleCI expected 3434; GitHub Actions expects 1001. This is the
reason zulip-ci.yml currently needs to mess with the permissions in
/__w.
Signed-off-by: Anders Kaseorg <anders@zulip.com>