Support python 3.9 and drop support python < 3.2

python 3.9 removes getiterator() of classes ElementTree , which are obsoleted from python 3.2: https://docs.python.org/3.9/whatsnew/3.9.html#removed

Change getiterator() to iter() introduced in python 3.2, and because of this, drop support python < 3.2.

Fixes #33 (closed)