model: Fix hypothesis integration with attr < 21.3.0

When using attr < 21.3.0, adding field transformer breaks attrs
integration with hypothesis, because attributes transformed with
such function are not casted to generated AttrsClass, but remains
just an list of attributes. This causes error in hypothesis by
raising an AttributeError.

As we use attr 21.2.0 in production and when building debian buster
package, add a workaround for that issue as explained here:
https://github.com/python-attrs/attrs/issues/821.