#14360: make encoders.encode_quopri work.

There were no tests for the encoders module.  encode_base64 worked
because it is the default and so got tested implicitly elsewhere, and
we use encode_7or8bit internally, so that worked, too.  I previously
fixed encode_noop, so this fix means that everythign in the encoders
module now works, hopefully correctly.  Also added an explicit test
for encode_base64.
Merge #14360: make encoders.encode_quopri work.
Issue #17914: Use os.cpu_count() instead of multiprocessing.cpu_count() where
applicable.
#18111: Add What's New entry for max/min default.
Improve imap error message in unusual failure mode.

We ran into this during the sprits at PyCon and this patch has been
sitting on my disk ever since.  This just adds some information to the
error message that we found useful during debugging.  There's no good
way to add a test, since the message only got generated via code
that we had modified for debugging purposes.
#14360: Add news item.
Merge #14360: Add news item.
Added example to enum docs show access to name and value attributes of enum members.