Ensure we build in python3
Run only small tests when building the package
Add correct dependencies and run tests
Ignore pybuild artefacts
Fix tests and runtime to run under root.

* sbuild runs under a root user, without SUDO* env variables. Support that case
* Fix tests in accordance to it to make all of them passing under sbuild
Update TODO
Add tests for switch_user()
First pass at autopkgs tests
Install build-deps for non virtualenv docker tests

* As we'll get tests running inside the docker container but without
  virtualenv, enable udtc to create a package containing all build-deps and
  install that in the container itself.
Ensure local udtc helper is using the local lib
Update build-deps
Switch for commands in the container using venv

We only use virtualenv if the external environment is using virtualenv
Enable running tests from local udtc or system one

* We can now run tests with local or system udtc binaries and libraries.
* Adding a -s switch to use the system one.