Cannot configure the Sphinx nitpick_ignore option

[jgosmann]

nengo-bones enforces that the Sphinx configuration is also created with nengo-bones and enables nitpicky by default. However, the Nengo SPA documentation references certain Nengo types in automatically linked fields that cannot be resolved (e.g. nengo.base.NengoObject which is not included in the public Nengo API documentation, but the appropriate type to use in the Nengo SPA documentation). These could in theory be ignored with the nitpick_ignore option, but this option requires a list of tuples which cannot be specified in the YAML file (YAML only has lists, but not tuples, a list instead of a tuple does not work with Sphinx).