We add `v3_1.py` and adapt the version number wherever we found.

This commit adds tests for `v3_1.py`.

When changing the module from `v3` to `v3_1`, the name obviously
got two characters longer.
This resulted in overstepping the maximum line limit in two
occasions.
We fix this oversight with this commit.

A new enumeration value has been added to class `Asset_kind`, see
[aas-specs#294](admin-shell-io/aas-specs#294)

Fixes #302

Previously, the `matches_id_short` regular expression only allowed
letters, digits and underscores.

With the decision of
[aas-specs#295](admin-shell-io/aas-specs#295),
this condition was relaxed to allow also hyphens.

Fixes #304

Previously, `Entity.entity_type` was mandatory, however v3.1
changes this.
We adapt the class defintion, as well as the invariant of AASd-014.

Fix #306