Change != to is not in pshell initialization

[ztane]

I had a curious problem: I had thrown an object that was wrapped in lazy_object_proxy added into the pshell environment. It had worked before but for some reason stopped working here merely due to updates in either Python version or some of the dependent libraries. The error thrown was AttributeError: __wrapped__ on this line. As is not is likely the more correct solution here, here's the pull request.