setup.py: Add more metadata useful for PyPI

- setup.py: List useful links using `project_urls`

- setup.py: Specify minumum python vresion using `python_requires`

  `python_requires` is the new way of specifying requirements for python
  version, as documented @
  https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires.

  This will also make this requirement appear nicely on the PyPI project
  page.

- setup.py: Add project classifiers

  PyPI will use these classifiers to categorize projects while searching
  or browsing. Full list of classifiers can be found at
  https://pypi.org/classifiers.