Update setup.py script

This updates the setup.py script so that it will work as it should.

Other noteworthy changes in this branch are:

* Less strict dependency on libdnet and pypcap

* Removing on txscapy as a url from requirements.txt as it's now in pypi

@ioerror I have update pypcap with a patch to it's setup.py script so that it does not break out of the setuptools sandbox. Could you update the version of pypcap that is hosted on https://people.torproject.org/~ioerror/src/mirrors/ooniprobe/* to this one: https://github.com/hellais/pypcap/archive/v1.1.1.zip?