Fix Python 3.8 compatibility (#6154)

rest_framework/compat.py

@@ -10,6 +10,13 @@
 from django.utils import six
 from django.views.generic import View
 
+try:
+    # Python 3 (required for 3.8+)
+    from collections.abc import Mapping   # noqa
+except ImportError:
+    # Python 2.7
+    from collections import Mapping   # noqa
+
 try:
     from django.urls import (  # noqa
         URLPattern,

rest_framework/serializers.py

@@ -15,7 +15,7 @@
 import copy
 import inspect
 import traceback
-from collections import Mapping, OrderedDict
+from collections import OrderedDict
 
 from django.core.exceptions import ImproperlyConfigured
 from django.core.exceptions import ValidationError as DjangoValidationError
@@ -27,7 +27,7 @@
 from django.utils.functional import cached_property
 from django.utils.translation import ugettext_lazy as _
 
-from rest_framework.compat import postgres_fields, unicode_to_repr
+from rest_framework.compat import Mapping, postgres_fields, unicode_to_repr
 from rest_framework.exceptions import ErrorDetail, ValidationError
 from rest_framework.fields import get_error_detail, set_value
 from rest_framework.settings import api_settings