Recent (>=3.4) versions of python cryptography now ship with rust code for some of the operations. This breaks in some scenarios (e.g. openstack-charmers/zaza-openstack-tests#369).

However, we do not want to remain pinned for long as it is a security related module. The main issue is rust builds for charms.reactive charms which needs a build for each platform. Or we need to NOT have the rust code.

Related: #420 pins to <3.4 in zaza.