[eliotwrobson]

Lets questions block a single library function with import_blacklist, instead of overriding it in setup code. Before, import_blacklist=["numpy.linalg.inv"] only blocked from numpy.linalg import inv, so np.linalg.inv(A) and la.inv(A) still worked.

• Startup: for each dotted entry whose parent is an importable module, and whose last part is a callable that isn't a module, the sandbox replaces it in that module with a function that raises RuntimeError("'numpy.linalg.inv' is blacklisted and cannot be used."). It does this before setup code runs, in both sandbox and workspace_sandbox.
• Other entries: module entries ("os", "matplotlib.pyplot"), constants and entries whose module isn't installed are left to the importer as before.
• Scope: modules are shared within the sandbox, so setup code and helper modules can't call the function either. The function's own module is imported at startup, which adds its import time to startup.

Ran the test suite inside the full grader image with this branch on PYTHONPATH. Before this change, the new scenario's np.linalg.inv and la.inv calls succeed.

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