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Sioc is needed for content negociation

Btw introduce SIOCT types

Otherwise, upgrades will fail as both packages will try to install
the same files.

Update the build system to use pybuild.

The new binary package with Python 3 support is named
python3-cubicweb-blog according to current standards.
Python 2 package is also updated to use the proper namespace
and a transitional package has been introduced to handle
the migration.

Fix MANIFEST.in to include a missing image, extra files
to run the tests and to not ship packaging files.

Support for Python 3.8 is disabled as it is currently broken
in Debian unstable.

- install deps from dev-requiremnts.txt file
- update pytest section
- update flake8 section

- update MANIFEST.in
- update cubicweb min version inside __pkginfo__.py
- use new style import

Thus avoiding default value of posargs in tox's test environment.

Otherwise, only test_blog.py was run.

By using non-released cubicweb-sioc which is now py3 compatible.
Also please note that reference of "cubicweb-sioc" was useless here because cubicweb-blog already depend on it.