add pypy to supported envs

[eltonlaw]

todo

1. Dockerfile.pybase builds the eltonlaw/pybase image which sets up the python environments. Add stuff to install pypy here.
2. Dockerfile builds the impyute image which installs dev requirements and installs impyute. Add stuff to install impyute into the new pypy enviroment.
3. Update README.rst to support pypy
4. Add $ docker run impyute pypy -m pytest to the test target in the Makefile

verifying things work

1. Rebuild pybase: image make rebuild-pybase
2. Rebuild impyute image: docker build -t impyute .
3. Check that $ docker run impyute pypy -m pytest runs. If tests are successful, it should be okay.