Skip tests for 24-bit wave file on big-endian platforms.

Skip tests for 24-bit wave file on big-endian platforms.

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.

This allows different parts of a program to use different methods for
starting processes without interfering with each other.

and groups() methods of match object now always return a string or a bytes
object.

Re #18521: fix not-quite-C syntax that works only because the PyXXX_Check are macros defined with () around them.