Issue #18999: Make multiprocessing use context objects.

This allows different parts of a program to use different methods for
starting processes without interfering with each other.
Fix import of SimpleQueue.
Close #19252: better test coverage for Enum.  Thanks, CliffM
#18891: Complete new provisional email API.

This adds EmailMessage and, MIMEPart subclasses of Message
with new API methods, and a ContentManager class used by
the new methods.  Also a new policy setting, content_manager.

Patch was reviewed by Stephen J. Turnbull and Serhiy Storchaka,
and reflects their feedback.

I will ideally add some examples of using the new API to the
documentation before the final release.