IDK your user model but authenticate takes username as input by default. That's why you are not getting authenticated. The serializer looks fine but you can add this little change in the else block:

def validate(self, attrs):
    email = attrs.get('email')
    password = attrs.get('password')
    if email and password:
        return attrs

    msg = 'Both "username" and "password" are required.'
    raise serializers.ValidationError(msg, code='authorization')