Support of Python 3.7+

[matrixise]

No description provided.

[nigelmegitt]

I'm using pyxb with Python 3.7.5 without any major issues.

Minor issues include stuff that's strictly not pyxb code, like the interface change for getting an xml serialised version of a document needs a bit of tweaking to get your choice of a binary stream or an encoded string, but that's probably an improvement.

[matrixise]

Thanks

[Hazedd]

i have some depreciation warnings on dependencies on 3.7. but i can feel the call pyxb should be maintained!