It's been possible to configure a key inline in the homeserver.yaml since
13bc1e07. Update `sign_json` to work with this.

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&gt;`>