feedback or volunteers welcome ;)

* improved exporting and importing extension objects to/from xml

* fixed a type annotation

* compability to python 3.7

* fixed type annotation for python < 3.9

* remove optional parameter for <=3.8

* added tests

* comp with old pyhton versions

* fixed an error in tests

* somehow the name of the struct makes a difference

* add feat: subscription.py optional param for sampling_interval

* fix: flake8 stuff

* fix: flake8 stuff

* add: missing typehints and small refactoring according to flake8

* Update subscription.py

* remove: unnecessary import

* update: comments

* update: fix wrong typehint

Fix typo in in XmlImporte._check_if_namespace_meta_information_is_added which results in an exception.

In case it was required after the import to still add an ns an exception occured. On normal flow the line `_check_if_namespace_meta_information_is_added`s  with the typo isn't executed and no test was available to test the behaviour.

* Added test for `_check_if_namespace_meta_information_is_added`
* Fix the type

The typing information tries to use an no existing dataclass, requires an forward reference (escaped with single ').

The depends_on function result is an unrestricted recursive call when the depency is it self.

For completness, these datatypes are abstract! If they are used in structs, abstract types are either Variant or ExtensionObjects.
If they only contain basic types (int16, float, double..) they are Variants.

This reverts commit 80b63f17.