Simplify and cleanup import

This commit pushes the import to the proper place and assumes that the
import will *not* break DRF compatibility prior to version 3.0.0.
Assuming the travis tests cover all prior supported versions and import
compat.py, the tests would catch this.

Author: mikebiglan

rest_framework_jwt/compat.py

@@ -1,4 +1,5 @@
 import rest_framework
+import rest_framework.serializers
 from django.forms import widgets
 from distutils.version import StrictVersion
 
@@ -9,9 +10,6 @@
     class PasswordField(CharField):
         widget = widgets.PasswordInput
 else:
-    # Mid-file import to potentially prevent issues occuring for DRF pre 3.0.0
-    import rest_framework.serializers
-
     class Serializer(rest_framework.serializers.Serializer):
         @property
         def object(self):