setup.py: add Django 2.1 classifier (#627)

Ref pypi/warehouse#4090

Author: michael-k

setup.py

@@ -44,6 +44,7 @@ def read(fname):
                  'Framework :: Django :: 1.10',
                  'Framework :: Django :: 1.11',
                  'Framework :: Django :: 2.0',
+                 'Framework :: Django :: 2.1',
                  'Intended Audience :: Developers',
                  'License :: OSI Approved :: BSD License',
                  'Operating System :: OS Independent',