Update pypi.python.org URL to pypi.org

For details on the new PyPI, see the blog post:

https://pythoninsider.blogspot.ca/2018/04/new-pypi-launched-legacy-pypi-shutting.html

Author: jdufresne

setup.py

@@ -13,7 +13,7 @@
     author='Rob Hudson',
     author_email='rob@cogit8.org',
     url='https://github.com/jazzband/django-debug-toolbar',
-    download_url='https://pypi.python.org/pypi/django-debug-toolbar',
+    download_url='https://pypi.org/project/django-debug-toolbar/',
     license='BSD',
     packages=find_packages(exclude=('tests.*', 'tests', 'example')),
     python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",