Fixes #1180

OPC-UA specification Part 4, 5.6.3 specifies that a Null or empty user
token shall always be interpreted as anonymous. Add a test for this case
and a fix to properly handle it.

do not print a very long exception trace when a handle does not have status_change_notification method

Certificates, that are already loaded into memory as bytes, can now directly be handed over
to set_security and load_certificate.

The parameter is not an instance of ua.SecurityPolicy, but a subclass of ua.SecurityPolicy.
This change satisfies the type-checker in PyCharm.

Default value should be False

Datavalues shall be set to Null if the status code indicates an error.
Previously it did so for anything that does not have status code good,
i.e. also uncertain status, which is only a warning.

* proposal-for-better-naming

* first shot on an ABC

* add missing and used session methods from nodeclass

* add ABC inheritance to InternalSession-Class

* update imports and typing

* make abstract methods async

* add missing methods in InternalSession

* rename new_node - server to session

* update InternalSession

* add info log for node registration

* rename server to session and add methods to interface

* add missing session methods

* fix typo

* rename transfer_subscription

* add comments

* add todo comment for BrowseNext

* renaming to session

* fix skipp tests on windws

* remove log in transfer_subscriptions

* remove log in browse_next

* remove log in transfer_subscriptions

* 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