setup.py: remove Django 2.1 classifier to fix pypi upload

Author: blueyed

setup.py

@@ -44,7 +44,6 @@ 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',