Incompatibility with nbconvert 6

[Cadair]

When running with nbconvert 6 installed, I get

AttributeError: 'ExecuteCoveragePreprocessor' object has no attribute 'setup_preprocessor'

irrespective of the value of the coverage setting.

[chrisjsewell]

Yeh should look to move to https://github.com/jupyter/nbclient
Also jupyter/nbclient#79 may be useful

[chrisjsewell]

Now fixed in pytest-notebook 0.7.0 (nbconvert is no longer a dependency)