While this is a pain for content developer local environment updates it breaks the LMS in EC2 which is vastly more important.  We'll jsut have to address helping individual content people with installing MySQL if needed. This is a real dependency in production and I think those belong in here.

pin requests to a known version, as we believe there's been an upgrade which is incompatible to our usage

We had this before as a python dependency but it was dropped
when we switched to virtual environments

Taking out MySQL-python for now because it requires mysql to be installed, so breaks updates on content folks' envs.

- needed for chemical equation parsing, which is about to appear in capa

Added django-wiki's requirements to requirements.txt, because they weren't being included automatically.

updating dependencies for pystache, networkx, graphviz and removing redundant dependency for django-staticfiles