[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.

Tests: unittest
Depends:
 python3-cubicweb,
 @builddeps@,
 python3-pytest,
 python3-crypto, python3-psycopg2,
 postgresql, postgresql-plpython,
 firefox
Restrictions: allow-stderr, isolation-container