Fix imports for DRF 3.10 (#408)

Fixes #410
Fixes #411

testproj/snippets/serializers.py

@@ -1,8 +1,13 @@
+import rest_framework
 from decimal import Decimal
 
 from django.contrib.auth import get_user_model
+from packaging.version import Version
+if Version(rest_framework.__version__) < Version('3.10'):
+    from rest_framework.compat import MaxLengthValidator, MinValueValidator
+else:
+    from django.core.validators import MaxLengthValidator, MinValueValidator
 from rest_framework import serializers
-from rest_framework.compat import MaxLengthValidator, MinValueValidator
 
 from snippets.models import LANGUAGE_CHOICES, STYLE_CHOICES, Snippet, SnippetViewer