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