Update docs/hacking.rst to include required pytest plugins

[martijnthe]

The doc says:

Use the py.test command from the WeasyPrint directory to run the test suite.

This doesn't work unless these additional pytest plugins are installed as well.

[liZe]

Thanks for the report and the fix! I've used .[test] instead of listing the plugins, we won't have to change this if we use other pytest plugins in the future.