The current implementation of PythonCallable::GetNumArguments
is not exception safe, has weird semantics, and is just plain
incorrect for some kinds of functions.
Python 3.3 introduces inspect.signature, which lets us easily
query for function signatures in a sane and documented way.
This patch leaves the old implementation in place for < 3.3,
but uses inspect.signature for modern pythons. It also leaves
the old weird semantics in place, but with FIXMEs grousing about
it. We should update the callers and fix the semantics in a
subsequent patch. It also adds some tests.