Thanks Ethan :)

From my point of view this is LGTM, as long as:

* There's ReST documentation
* You remove the code to support extensions and customizations not mandated by PEP 435. As I mentioned before, this seems to be a YAGNI that complicates the code needlessly. It's find to keep the changes in some external repo and at a later point discuss their gradual addition similarly to the way Nick has been pushing enhancements through additional issues.

We can always add more capabilities to Enum. We can "never" take them away once added, and this complicated code will remain with us forever even if no one ends up using it.