python/pkgmanager.cc: Do not create temporary CppPyRef

Instead of creating a CppPyRef with the only purpose of
destroying it, just decrement the reference count.

Reported-by: cppcheck
tests/test_aptsources.py: Fix regression in d95a8f2

Commit d95a8f27774842beb43bda54c4c61b67fe76a032 introduced a regression
in this test that did not catch an invalid False value for the trusted
property of a source without trusted set.
tests/test_deps.py: test_dstr cleanup and fixes

Simplify the code by not stating all test cases in the
code, but using a loop. Also adds more tests this way.
apt/package.py: Improved readability and documentation of BaseDependency.__dstr().
apt/package.py: Pass an apt_pkg.Dependency to BaseDependency

Instead of passing the properties to __init__(), let's just pass
the apt_pkg.Dependency. Store that in a _dep attribute, and provide
properties to provide the API. The other classes do it this way
as well.

Reported-by: Michael Schaller <michael@5challer.de>
debian/control: Set Standards-Version to 3.9.5
debian/control: Drop python-{unittest2,debian} build deps

They are not used anymore.
Release 0.9.2
doc/source/conf.py: Python 3 support

Correctly decode the process output in Python 3, so everything
works if sphinx is run using python3.
debian/rules: Use dh_sphinxdoc integration

This ensures that we do not include JavaScript from
Sphinx in our binary package. Adjust debian/control
to include the dependencies created by dh_sphinxdoc.