Python 3.10 support

[blayzen-w]

It looks like this does not work with python 3.10. It raises an AttributeError 'xml.etree.ElementTree.Element' object has no attribute 'getchildren'. This seems to be caused by the deprecation and removal of the ElementTree.getchindren function (deprecated in 3.2, removed in 3.9)

See https://docs.python.org/3.8/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.getchildren

[blayzen-w]

It looks like a lot of the unit tests are failing due to the USPS api requiring the data to be submitted in an exact order. I'm going to do a bit of a rewrite to fix the problems and get things up to date. I hopefully should have a PR up soon. If you need help maintaining the project, I'm happy to help.

[blayzen-w]

I setup github actions on our fork. Let me know if you want those too and I can open a pull request. It just requires you to add a repository secret for the USPS access key to work.