text: Title: nbval not working with parallel/distributed pytest
  Body: Parallel/distributed testing with pytest (pytest-xdist) fails: it seems that all cells are executed in parallel instead of sequentially, which will obviously not work for most notebooks.
  
  To reproduce, create a notebook doing dependent things in different cells, for example import a package in the first cell and then use it in other packages. Install pytest-xdist. Then, try for example:
  
  ```shell
  purest -n auto --nbval <notebook file>
  ```
  
  (the `-n auto` option asking pytest to use all available CPU cores on the machine)