From code review: the hash was shallow, so nested objects could have hashed differently when they didn't need to.

Change the XBlock target hash, not because we need new code, but because the old one was tangled up in a eponymous branch that was causing trouble.

adding pre-requirements for sandbox-requirements.txt

Fix UnicodeEncodeError when generating cache keys

from non-ASCII unicode code submissions.

A new boolean on XModuleSystem that determines whether to allow execution of untrusted unsandboxed code.

Python should have -E, not sure of a clean way to do it, but this at least only applies it to python.