frome: Add a dependency on python3-apt

The frome test currently fails with the message:
  UserWarning: Parsing of Deb822 data with python3-apt's apt_pkg was
    requested but this package is not importable. Is python3-apt installed?

python3-debian only recommends to to install python3-apt but does not
depend on it.

Forcing installation.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>