to list exactly which file descriptors should be kept open.

not passed to subprocess.Popen as the default value will be changing in a
future Python to the safer and more often desired value of True.

DeprecationWarnings that show up in a lot of existing code are controversial
and have caused pain in the past.  I'd like to leave this on for 3.2 beta1 and
see how things go.  We can remove the warning if it is deemed too noisy during
any betas.  (case study: the md5 and sha module DeprecationWarnings are loathed
around the world as those modules were never going to be removed in 2.x and
2to3 has a fixer for code that uses them)

Added a few common Popen uses to the tests like we've done for a few other
instances of adding context managers. Eventually the entire test suite
could be converted to use the context manager format.

Rather than list out the three signals (or more over time), the message was
made less specific but still descriptive.