setup: bump formencode minimum version to 1.3.0

The formencode version range included both 1.2.x and 1.3.x releases.
However, since 1.3.0, _to_python and validate_python are deprecated and
renamed to _convert_to_python and _validate_python, respectively.

With current pytest, these (long) deprecation warnings are shown in the test
logs.

There are two options:
- restrict maximum version to 1.2.x
- bump minimum version to 1.3.x

In this commit we choose the latter approach, going towards the future
rather than the past.