Bug: PyPy does not have ENOTSUP

[dargueta]

To reproduce: Run the unit tests on PyPy. They fail because errno doesn't have ENOTSUP defined, even though it's present on the same platform for CPython.

We can fix it by using the CPython value as a default. Error codes aren't guaranteed to be identical across all platforms so this might not work everywhere, but it'll prevent the crash.