Check that all context managed objects are properly closed

Check that all context managed objects are properly closed

Ensure all of our context managed objects have a __del__ that asserts
that the instance has been properly closed so that we'll be more
likely to notice related oversights.

This will only work in cases where __del__ is called before shutdown,
but that should normally be the case for cpython.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>