Bump debhelper from deprecated 8 to 12.
+ Replace python_distutils buildsystem with pybuild.
+ debian/rules: Rely on default use of dh_missing rather than using dh_install --list-missing.
+ debian/rules: Replace dh_clean -k with dh_prep.
Fixed-Lintian-Tags: package-uses-deprecated-debhelper-compat-version
