Style fix: Do not use "is True" / "is False"

This is uncommon style. It's better to use "pred" instead of "pred is True"
and "not pred" instead of "pred is False".
debian/control: Replace a tab with spaces

This looked a bit ugly if you had the wrong tab size.
Drop support for Python 2.6

This will allow us to use more Python 3 idioms in the
code, and possibly even get a single code base that works
in Python 2.7 and Python 3.3 and newer.
tests: Remove checks for Python older than 2.7

This simplifies the test code a tiny bit.
Use print_function everywhere

This brings the code closer to Python 3. Automatically converted
using 2to3 -f print.
Use a single code base for Python 2 and 3

This is much better than running 2to3 during the build, as it
gives us more control over the Python 3 code.
apt, doc: Fix the list of comp_type values

Reported-by: Michael Schaller <michael@5challer.de>
Unify list of comparison type values everywhere

They are now ordered and include the != and < operators
in all cases.
apt/package.py: Fix BaseDependency.__dstr.__ne__()

It turns out I missed this. Just simplify it to return not
self.__eq__(other). Also add a test case for it.

Reported-by: Michael Schaller <michael@5challer.de>
tests/test_deps.py: Remove an invalid blank line

Fixes pep8
debian/README.source: Drop it, it's useless bzr

This contains some bzr advices. It's wrong for git, and we don't
need it for git because git is popular.
tests: Add back test_hashsums from old and update

These are automatic tests and they still work, so it's pointless
not to use them.
debian/gbp.conf: Set sign-tags = True
apply patch from Barry Warsaw to fix autopkgtest
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