Issue #16954: Add docstrings for ElementTree

Based on patch by David Lam
Touch up the Python 2 to 3 porting guide
merge
#11963: avoid printing messages in test_parser.  Initial patch by ?ric Araujo.
#11963: merge with 3.2.
#11963: merge with 3.3.
#11963: fix Windows buildbots.
#11963: merge with 3.2.
#11963: merge with 3.3.