Issue 20123: Fix pydoc.synopsis() for "binary" modules.

Also add missing tests to test_pydoc.
whatsnew: XMLPullParser, plus some doc updates.

I was confused by the text saying that read_events "iterated", since it
actually returns an iterator (that's what a generator does) that the
caller must then iterate.  So I tidied up the language.  I'm not sure
what the sentence "Events provided in a previous call to read_events()
will not be yielded again." is trying to convey, so I didn't try to fix that.

Also fixed a couple more news items.
Issue 20123: try using a different builtin module in a pydoc test.

The test is failing on one of the stable FreeBSD buildbots.  It seems
unlikely that the gc module would not be available, so switching to
_imp may not fix the problem.
Fix news entry to use updated name for XMLPullParser.
Issue 20123: Disable a problematic test.
Argument Clinic: fixed test suite, improved howto.
Regenerated pydoc/topics.py, and fix a "suspicious" doc error.
Bump version number for 3.4.0b2.
Added tag v3.4.0b2 for changeset ba32913eb13e
Issue19995: %o, %x, %X now only accept ints
Add comments to frozenset_hash().

Also, provide a minor hint to the compiler on how to group the xors.
Post-release engineering; updated NEWS and version string.
Merge 3.4.0b2 release revisions back into mainline.
Issue #20142: Py_buffer variables generated by Argument Clinic are now
initialized with a default value.
Issue #20143: The line numbers reported in Argument Clinic errors are
now more accurate.
whatsnew: pydoc.Scanner removal, check_output input parm, operator.py.

Also fleshed out the entry on struct.iter_unpack.
whatsnew: ssl getpeercert/do_handshake raise OSError, weakref __callback__.

Also add a missing word to gc entry, and delete a now-obsolete doc note
in the weakref __callback__ docs.  (Opened an issue for rewriting
the section that compares finalizers and __del__ method.)
asyncio: Fix deadlock in readexactly(). Fixes issue #20154.
Remove dead PEP 451 code.
Issue #19703: Update pydoc to use the new importer APIs.