Prior to this commit, we useda custom modified versin of python-zeep
that accepts a list of headesr as part of the _soapheader argument.
Recent changes to python-zeep include that support natively using the
_soapeheaders [plural] argument. Port the code to that.
The 1C4A hash is computed based (among other things) the current German
time. Let's use German time no matter which timezone the executing
system is using.