PyXB is EoL, Python 3.8 Support

[nickpresta]

Hi,

This library uses the pyxb package, which is marked as "end of life" by the maintainer.

Similarly, the pyxb package doesn't work in Python 3.8, and therefore this package doesn't work in Python 3.8.

I am submitting this issue as a request to support Python 3.8, by:

• Monkeypatching pyxb so consumers don't have to workaround internal implementation details of this package.
• Switching to something other than pyxb

Thanks