Fix 1.10 deprecation warnings

[rpkilby]

Sorry to recreate #4488 - had to switch branches.

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Description

Fixes a number of deprecation warnings generated by DRF. The to-many fix is the only one that really concerns me, as it makes a call to model_meta.get_field_info. Not sure if there's a better way to pass that information to the create/update methods.

There isn't really an easy way to test this without manually running the tests with the -Werror flag. That said, django-guardian also generates a number of warnings too. Just to test it out, you can install my fork, which has the fixes applied.

[rpkilby]

Adding comment from original PR:

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btw - there are two remaining deprecation warnings regarding the use of include:

• tests/test_routers.py:61
• tests/test_versioning.py:166

RemovedInDjango20Warning: Specifying a namespace in django.conf.urls.include() without providing an app_name is deprecated. Set the app_name attribute in the included module, or pass a 2-tuple containing the list of patterns and app_name instead.

Not sure how to fix these, and tbh I'm not too worried about it as it's internal to the test suite.

[rpkilby]

btw, this test did not fail despite the urls property having been fully deprecated in 1.10

[lovelydinosaur]

Now merged into version-3-5 branch. (#4525) Thanks!