Validators on field sets are ignored with zope.interface 5.1.x

[mauritsvanrees]

Seen with Plone 5.2.2, Python 3 (I guess Python 2 as well), with zope.interface updated from 5.0.2 to 5.1.2. Same for 5.1.0 or 5.1.1. Same for Plone 5.2.3-pending.

• Create a standard easyform.
• Remove the standard fields and add one non-required field (or remove subject and comments, and make the email field non-required).
• Add a fieldset.
• Add a field in this field set and make it required or add a validator.
• View and submit the form. If the field in the fieldset is required, do not fill it in. If it has a different validator, fill in something that should not validate.
• Expected is a validation error, but instead you get a thank-you page. Or if the form has a Mailer, you may get an error because the Mailer cannot find the email.

The changes from @djay in PR #190 last year may have something to do with it. At least the PR points to the parts of the code that may need new changes to fit the updated zope.interface.
I suppose it is possible that this fix is no longer needed with the newer zope.interface: it has cleaner inheritance, so maybe it now works as expected without needing extra special code. But that may be wishful thinking.

[mauritsvanrees]

I confirm that the changes from @djay from last year are still needed. They only need to be slightly adjusted. I have done so in my PR #253.