Declare a different name for nan_equality.UNEQUAL to prevent Cython warnings.

[bdice]

Description

Closes #9462.

warning: cudf/_lib/cpp/types.pxd:51:8: 'UNEQUAL' redeclared

We've been dealing with warnings like this for as long as I've worked on cudf. I think it'd be good to fix this by renaming the Cython enum. We don't rely on this name very much at the Cython/Python level, so it's a small change. (This has been irking me for a long time and I want to see this warning go away in the build logs.)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

/merge