AttributeError when converting to h5 with Python 3.9

[mike-welch]

I encountered an AttributeError on this line:
https://github.com/NREL/h5plexos/blob/33dc07a6b1ffe47574ad5cf73c5243584508a15d/h5plexos/process/tempdb.py#L116

The Element.getchildren method was removed in Python 3.9, see:
https://docs.python.org/3.8/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.getchildren

As the docs suggest, using list(elem) instead allows the code to run.

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EDIT I later encountered an error in h5py so YMMV.