Close #19011:  Add documentation on some of the non-standard Enum behavior.

Issues such as iterating and containment on Enum classes, why Enum member
instances do not show up on other Enum members, and how __new__ should
be used.
Issue19030: fixed comment that was still referring to a changed descriptor.
Close #18990: remove root attribute from XMLPullParser

- this was an internal implementation detail for iterparse
- this has been changed to use a new private method instead
- XMLPullParser.close docs are now more explicit about not
  returning a root element and instead direct users towards
  read_events
- also added missing docstrings and clarified some details
  related to exactly *when* events are consumed from the
  internal queue

(Initial patch by Stefan Behnel)
Close #18596: Support address sanity checking in clang/GCC

This patch appropriately marks known false alarms in the
small object allocator when address sanity checking is
enabled (patch contributed by Dhiru Kholia).
Issue #18950: Fix miscellaneous bugs in the sunau module.

Au_read.readframes() now updates current file position and reads correct
number of frames from multichannel stream.  Au_write.writeframesraw() now
correctly updates current file position.  Au_read.getnframes() now returns an
integer (as in Python 2).  Au_read and Au_write now correctly works with file
object if start file position is not a zero.
Issue #18950: Fix miscellaneous bugs in the sunau module.

Au_read.readframes() now updates current file position and reads correct
number of frames from multichannel stream.  Au_write.writeframesraw() now
correctly updates current file position.  Au_read.getnframes() now returns an
integer (as in Python 2).  Au_read and Au_write now correctly works with file
object if start file position is not a zero.
fix duplicate test names (closes #19115)

Patch by Xavier de Gaye.
merge 3.3 (#19115)