move import of soapbox.xsd (in soapbox.utils) into method as the utils package from my point of view provides basic utilities for all other modules. With a class-level import of "xsd" that module can not use any functionality from the utils module.
fix timezone offset formatting for xs:datetime.

Sigh, how could I assume I get this right without proper tests? Stupid me.
move tests for xs:datetime into separate file
fix UTC offset calculation for xs:datetime.

Another stupid bug with didn't get enough unit tests so it could slip through.