Drop python3-mox dependency

It was only used by sso-server, which is not being built. This
package is not available in Debian buster.

debian/control

@@ -5,7 +5,7 @@ Maintainer: Autistici/Inventati <debian@autistici.org>
 Build-Depends: debhelper (>= 10), apache2-dev | apache2-prefork-dev | apache2-threaded-dev,
  apache2, autoconf, automake, libtool, python3-dev, dh-python, python3-all,
  libpam-dev, libssl-dev, python3-setuptools, pkg-config, libz-dev,
- python3-requests, python3-nose, python3-mox
+ python3-requests, python3-nose
 Standards-Version: 3.7.2
 
 Package: ai-sso