Sphinx 3 issues

[effigies]

Tracking thread for Sphinx 3 upgrade.

• plot_workflow directive is failing:

   Creating file /tmp/src/nipype/doc/users/examples/smoothing_comparison.rst.
  
   Exception occurred:
     File "/usr/local/lib/python3.7/site-packages/nipype/sphinxext/plot_workflow.py", line 256, in setup
       app.add_directive("workflow", wf_directive, True, (0, 2, False), **options)
   TypeError: add_directive() got an unexpected keyword argument 'alt'