Update references to parent organization

The repository has been transfered from the dropbox org to the python
org on Github.

Author: ddfisher

CONTRIBUTING.md

@@ -1 +1 @@
-To contribute code to this project, you'll need to sign [Dropbox's Contributor License Agreement](https://opensource.dropbox.com/cla/).
+To contribute code to this project, you'll need to sign the [Python Software Foundation's Contributor License Agreement](https://www.python.org/psf/contrib/contrib-form/).

setup.py

@@ -89,7 +89,7 @@
        long_description = long_description,
        author = 'David Fisher',
        author_email = '[email protected]',
-       url = 'https://github.com/dropbox/typed_ast',
+       url = 'https://github.com/python/typed_ast',
        license='Apache License 2.0',
        platforms = ['POSIX', 'Windows'],
        classifiers = [