I found the cause of the failed assertion, an invalid escape sequence slipped through in a file. Patch attached (also with Serhiy's comments).

It worries me a little though that pure Python code can cause a hard crash. Ok, it worries me a lot. Please don't merge this until it's fixed. I'm guessing this is a combination of unittest catching warnings and compiling the faulty source file. As to why a malformed node (i.e. one that raised a DeprecationWarning) managed to pass through unharmed is beyond me.