`fields.Constant` with `required=True` flag raised `load_default` warning

[nosnickid]

I have a schema containing a Constant, with required=True set, as the schema is ultimately exported to an OpenAPI spec, and the field must be marked as required there.

The fix from #2894 now triggers the validation failure in

marshmallow/src/marshmallow/fields.py

Line 216 in 2a3812d

if required is True and load_default is not missing_:

My feeling on the fix is to undo the changes from #2894 and instead reproduce the allow_none flag initialization in Constant.__init__

Happy to create a PR if you'd be happy with this fix.

[sloria]

Thanks for reporting! Would happily review and merge a PR