Before, the `make_label` function had a `target` parameter
that had to be a `LinkSub`, `[Part::Feature, "Edge1"]`.
However, it is easier to pass the individual components
separately using two parameters, `target_object` and `subelements`,
which can then be assembled into `[target_object, subelements]`.
The second element is optional, so it can be set to `None`,
and then the second element will be converted to an empty list,
`[target_object, []]`, as required by the Label's underlying `Target`
property.
Also perform several improvements such as PEP8 cleanup,
writing complete docstrings, type checking the input arguments,
and deprecating the older call.
Update `Draft.py`, the Gui Command, the unit test, and test script
as well.
In Python 3 `super()` can be used without an argument
in order to get the parent class, `super()`.
In Python 2 the present class must be used as an argument,
`super(ThisClass, self)`.
This commit is done to support Python 2,
as without it all Gui Command tools will be broken
and will fail to launch, even if the unit tests pass.
Also, set the `__metaclass__` variable to `type`.
This is done to turn all classes into "new style" classes
in Python 2. This is required so `super()` works correctly
in this version of Pyton.
This commit can be reverted once Python 2 support
is completely dropped, and only Python 3 compatible code
is used.
