improve django version checking

Author: mehdipourfar

postgres_composite_types/forms.py

@@ -36,7 +36,7 @@
 import logging
 from collections import OrderedDict
 
-import django
+from django import VERSION
 from django import forms
 from django.contrib.postgres.utils import prefix_validation_error
 from django.utils.translation import ugettext as _
@@ -45,6 +45,8 @@
 
 LOGGER = logging.getLogger(__name__)
 
+DJANGO21 = VERSION >= (2, 1)
+
 
 class CompositeBoundField(forms.BoundField):
     """
@@ -145,7 +147,7 @@ def clean(self, value):
                 try:
                     cleaned_data[name] = field.clean(value.get(name))
                 except forms.ValidationError as error:
-                    prefix = '%(label)s:' if django.__version__ >= '2.1.0' \
+                    prefix = '%(label)s:' if DJANGO21 \
                              else '%(label)s: '
                     errors.append(prefix_validation_error(
                         error, code='field_invalid',