[pkg] Tidy substvars usage in control file The documentation package was missing a Built-Using field to record the provenance of the templates. Empty packages or meta packages do not require Depends on a specific Python version, so we can remove the relevant substvars.

[pkg] Specify priority “optional” instead of the obsolete “extra” Quoting Lintian, “Since Debian Policy version 4.0.1, the priority extra has been deprecated.” So let's use “optional” instead.