Issue #17901: fix TreeBuilder construction for an explicit element_factory=None

Based on report and patch by Aaron Oakley.
Issue #17901: fix TreeBuilder construction for an explicit element_factory=None

Based on report and patch by Aaron Oakley.
Issue #17937: Try harder to collect cyclic garbage at shutdown.
Issue #17812: Fixed quadratic complexity of base64.b32encode().
Issue #17812: Fixed quadratic complexity of base64.b32encode().
Optimize base64.b32encode() and base64.b32decode() (speed up to 3x).
Issue #11995: test_pydoc doesn't import all sys.path modules anymore.
Issue #11995: test_pydoc doesn't import all sys.path modules anymore.
Try to make test more reliable (saw some sporadic failures on buildbots)
Try to make test more reliable (saw some sporadic failures on buildbots)
_elementtree.XMLParser._setevents should support any sequence, not just tuples

Also clean up some code around this
Clarify docs too: events can be any sequence (not that the C code supports it)
Issue #17988: remove unused alias for Element and rename the used one

Renaming to _Element_Py for clarity and moving it to a more logical location.
_ElementInterface OTOH is unused and is therefore removed.

Close #17988
Cleanup more old ET library leftovers
Cleanup even more dead code
normalize whitespace