Consider replacing collections OrderedDict by plain dict

[pombredanne]

There are still many places where OrderedDict are used https://github.com/encode/django-rest-framework/search?q=ordereddict
Since the minimal version is 3.6, these could be replace by a plain dict which is now ordered: using dict is overall faster and more concise.

[realsuayip]

@carltongibson it has been a while since #7524, do you think this is plausible at this moment?

[carltongibson]

As per the comment there, after 2.2 was EOL it's totally doable