Avoid O(n) lookup when creating cuDF Python mixins

[mroeschke]

Description

Minor, but __init_subclass__ would do a O(n) lookup in a loop, operation in dir(cls), where dir(cls) would increase as we setattr.

We can set the original dir(cls), and check each operation against that since we know all added operations will be unique.

Checklist

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

[mroeschke]

/merge