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This MR merges feature/xml_deserilization into master. It comes with deserialization tests and serialize + deserialize tests, just like the json module.

There's just one thing left to discuss, the problem I also mentioned in yesterday's meeting:
I'm using a TypedDict to pass the modeling kind of submodels and submodel elements to the respective constructor only when it's specified in the xml document, see _get_modeling_kind_kwarg(). I'm doing it this way because in my opinion defining the default modeling kind should be a task of the respective model constructors, not of the deserialization, since the default value is also only defined in the model part of Details of the Asset Administration Shell Part 1 (see chapter 4.7.2.3), not in the serialization/deserialization part.

The problem is, that I'm importing TypedDict from mypy_extensions, because it's only available from typing in python 3.8+. mypy is not a runtime dependency, thus I can't use mypy_extensions in code that is not for testing only.
An alternative would be to also just define the default modeling kind in the xml_deserialization, just like the json_deserialization does.

I'd like to hear your opinion on this, maybe you can come up with other alternatives.

The rest of this merge request is ready for code review btw.