Add a new warning gategory, ResourceWarning, as discussed on python-dev.  It is silent by default,
except when configured --with-pydebug.

Emit this warning from the GC shutdown procedure, rather than just printing to stderr.
diff --git a/Lib/test/exception_hierarchy.txt b/Lib/test/exception_hierarchy.txt
index 73ccb66..5037b33 100644
--- a/Lib/test/exception_hierarchy.txt
+++ b/Lib/test/exception_hierarchy.txt
@@ -47,3 +47,4 @@
            +-- ImportWarning
            +-- UnicodeWarning
            +-- BytesWarning
+           +-- ResourceWarning