Fix email constraint. InvalidConstraint need a model field, not a string

[qtdzz]

A small and very rare bug when generating user model.

[gregmuellegger]

Hi thanks for the proposal. Can you describe very quickly when you hit this bug? I don't get yet what this PR is changing in the semantics.

[qtdzz]

Hi,
The bug happens when the generated email is not unique. Actually it is very rare (or impossible) to reach this bug. I recognized it when I referenced "unique_email" to make my own constraint. But when my constraint is not satisfied, it was crashed instead of raising InvalidConstraint.
I took a look at the constraints.py and found that it takes a model field as parameter, not a string.
Thanks for your consideration :)

[gregmuellegger]

Oops, I didn't see that. This went unnoticed since 2010 :) Thanks for the fix and the explanation.

[gregmuellegger]

Do you need a new release with this fix urgently? Otherwise it will just be part of the next random release :)

[qtdzz]

Hi,
I think you don't need to make a new release because it may never reach that bug. It is just helpful for people who refer that point to make their own constraints 👍