Need to pin cython version in requirements.txt

[Permafacture]

pip install extractnet failed for me because setup.py requires cython installed. I downloaded and pre-installed requirements.txt and then pip install extractnet failed because of an error with cython. I saw a warning that said some default had changed since previous releases so I looked up what cython was most recent when the requirements.txt was written.

pip install "Cython==0.29.34" fixed my issue

Note I also had to apt-get install libxml2-dev to get this to install