Replace usages of `rez.vendor.enum` with the built-in enum module

[JeanChristopheMorinPerso]

Now that we dropped support for Python 2, we can now use the built-in enum module.

Note that this doesn't apply to things inside rez.vendor. But if rez.vendor.enum is not used inside rez.vendor, we should remove rez.vendor.enum entirely.

[predat]

Hi @JeanChristopheMorinPerso, for me, I don't see any good reason to keep rez.vendor.enum even for pika; I would drop it completely.