override dnspython and pypdf

dns python has separate stable releases for 2 and 3, but they haven't specified classifier in PyPi. Details here http://www.dnspython.org/

It also overrides `pypdf` as there is another package called `pypdf2` https://warehouse.python.org/project/PyPDF2/ endorsed by original creator of pypdf.
This should actually be https://pypi.python.org/pypi/dnspython3/ as that's the project that implements Py 3 compatibility