Regression test suite now passes.  =)  Had to add the support for when
warnings.showwarning() is set and a bug in PyErr_WarnExplicit() where a
NULL value for the module name was not being allowed.

Couple things still left to implement.  One is the second output line in
show_warning().  Don't notice this in unit tests as it imports warnings
and thus uses the Python implementation that works properly.  Also need
to implement warn_explicit().  Lastly, -W arguments need to be handled.

In terms of differing semantics, file paths are different.  Old way did
absolute paths.  It also handled what the file path should be when
executing a warning from the interpreter differently.

In terms of testing, some _warnings-specific tests are needed.  As
mentioned above, the differences between _warnings.show_warning() and
warnings.showwarning() are not being picked up.  This shows how tests
for setting of 'filters', 'onceregistry', and 'showwarning' needs to be
tested.

Assigning to Neal to see if there is anything I missed in terms of todos
and see if he wants to take any of them on.  =)