Move dependency on `testfixtures` module 
Move dependency on `testfixtures` module 

Fixes https://github.com/mwouts/jupytext/issues/64
mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext