[pkg] require python >= 3.4
This avoid pip pulling a version that does not run on python2 when using a
python2 environment.
Since we already released some 3.27 releases in pypi, I think we should release
3.27.3 and remove releases 3.27.2, 3.27.1 and 3.27.0 from pypi.
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo