sign_json
It's been possible to configure a key inline in the homeserver.yaml since 13bc1e0746. Update `sign_json` to work with this.
13bc1e0746
Part of #9744 Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now. `Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>`