Pytest the entire repo (#980)

* Pytest the entire repo

* Do each directory for now...

* YAML files hate tabs

* Add more requirements

* pip install opencv-python

* Comment out FTP

* Add pandas and sklearn to requirements

* Comment out FTP, graphs, machine_learning, maths, neural_network, project_euler

* Update .travis.yml

* Comment out Data structures

* if __name__ == "__main__":

* pytest --ignore=

* pytest .

* Update .travis.yml

* pytest . --doctest-modules --ignore=${IGNORE}

* Ignore --ignore because it just hangs