Missing support for the s390 architecture

[CarstenLeue]

https://github.com/upciti/wheel2deb/blob/master/src/_wheel2deb/debian.py#L27

I suggest to add the following architecture mapping to support s390 (linux on Z)

'linux_s390x': 's390x'

Where 'linux_s390x' is the python identifier for the architecture and s390x the debian identifier.