add GitHub URL for PyPi (dropbox#417)

Co-authored-by: Scott Erickson <sderickson@gmail.com>

Author: andriyor

setup.py

@@ -61,6 +61,9 @@
  license='MIT License',
  long_description=README,
  url='http://www.dropbox.com/developers',
+ project_urls={
+ 'Source': 'https://github.com/dropbox/dropbox-sdk-python',
+ },
  # From <https://pypi.python.org/pypi?%3Aaction=list_classifiers>
  classifiers=[
  'Development Status :: 5 - Production/Stable',