[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.
bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')