Fix test failure with wand 0.4.1+ wand 0.4.1 added an Image.destroy() (https://sources.debian.net/src/wand/0.4.4-1/wand/image.py/#L2760) that iterates over self.sequence (the frames of an animation) to free their memory; this throws an exception on single images (where self.sequence=None), but as this is called from __del__, this exception is warned about then ignored (https://docs.python.org/3/reference/datamodel.html#object.__del__), and hence is not an error in normal use.
However, blockdiag tests that use capture_stderr fail on any output containing "Traceback", including this warning message.
This patch ignores this message to allow blockdiag to build.
