https://github.com/fonttools/fonttools/pull/1335#issuecomment-431118568
only present in py27 elementtree: 6e57382464/Lib/xml/etree/ElementTree.py (L1676-L1677)
6e57382464/Lib/xml/etree/ElementTree.py (L1676-L1677)
we can use this ElementTree-like XML API for ttx or designspaceLib, not just ufoLib