[schema] use json to serialize constraints
Require yams 0.43: constraints are serialized to json, which means we
need to recreate the actual checks in the database on upgrade.
Temporary deps in tox.ini to pull respective changes in yams.
3.23 (UNRELEASED)
=================
Backwards-incompatible changes
------------------------------
* Generative tests *à la logilab-common* are not supported anymore in
`CubicWebTC`. It is advised to use the subtests_ API (available on
`CubicWebTC` either from the standard library as of Python 3.4 or through
unittest2 package otherwise).
.. _subtests: https://docs.python.org/3/library/unittest.html#distinguishing-test-iterations-using-subtests