[autoform] Make RelationField.fromcardinality a class method factory
Instead of having the "RelationField" class hard-coded in this factory
method (thus preventing any subclass to properly use it), we make that
method a classmethod and instantiate the actual class instead of the
base one.
version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
https://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))