test_crispr.py only works in certain environments

[tcmitchell]

test_crispr.py can fail if an older version of sbol2 is installed in the python interpreter. The way CRISPR_example.py is executed it runs in the python interpreter in a temporary directory. This means it doesn't have access to the module through the sbol2 directory in the current working directory.

When run with a virtual environment that has sbol2 installed in development mode (pip install -e .) everything will be fine. And on Travis it works because the current module is being installed in a clean environment.

This may not be much of a problem going forward, once we reach some stability. But it may be a problem for some developers depending on how their development environment is set up.