Also set up environment-related details needed for a smooth Cython
experience: have the test script avoid rebuilding all dependencies if the
virtualenv directory already exists, have the PyPI distribution script
distribute the Cython generated C code rather than the .pyx files.
Allows:
- running python tests in parallel
- clearer Travis output
- subjects each python test to the five minute run_tests timeout,
instead of ALL the tests to the five minute timeout
- easier benchmarking of which tests are slow