Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

from rest_framework import serializers
serializers.DecimalField(1, 1).to_internal_value('snan')

Expected behavior

Raises rest_framework.serializers.ValidationError with message A valid number is required. like for a 'nan' value.

Actual behavior

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/rest_framework/fields.py", line 1067, in to_internal_value
    if value != value:
decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]