Add support for overriding fields in serializer mixins.

[craigds]

This is an updated version of my old PR (#1053) applied to current master.

The implementation is a bit simpler now that OrderedDict is a thing.

[tomchristie]

Sorry - finding it a bit hard from the tests cases to see what the behavioral change is here.
Could you provide a simple example?

[craigds]

Sure

from rest_framework import serializers

class A(serializers.Serializer):
    myfield = serializers.IntegerField()

class B(serializers.Serializer):
    myfield = serializers.CharField()


class Sub(A, B):
    pass

# Original behaviour (wrong, confusing)
>>> Sub._declared_fields['myfield']
CharField()

# New behaviour (correct)
>>> Sub._declared_fields['myfield']
IntegerField()

[craigds]

Sorry, the title of the PR was slightly incorrect, probably why you're a bit confused. A case that actually works fine:

from rest_framework import serializers

class A(serializers.Serializer):
    myfield = serializers.IntegerField()

class Sub(A):
    myfield = serializers.FloatField()


>>> Sub._declared_fields['myfield']
FloatField()

I think that was also broken when I submitted my original PR years ago, hence the slightly confused title of this PR. It seems to have been fixed since then.

The diamond inheritance case is still broken, hence this PR.

[craigds]

Fixed title to match the actual fix

[craigds]

not sure why I closed this in 2016, but I have been maintaining a patch in our fork since then ...

Now upgrading to a modern DRF, I see it's fixed as of b8c369c (3.11+; #6980) 👍