Fix imports binding sys to a local

[evhub]

We need to use the actual variable name sys because mypy requires it, so our only real option here is to make sys an unredefinable built-in, like _coconut names, so that we can guarantee that the name sys will always globally refer to what we want and thus we don't have to redefine it locally.

[evhub]

We can even only allow import sys as the method of defining sys, only allow it at the top level, and enforce that any references to sys have to come after import sys.