PyGMT figures are not being showed in the latest docs

[santisoler]

Page that contains the error or needs to be improved (paste link):

The PyGMT figures we generate while building the docs are not being correctly added to the dev versions of the Harmonica docs.

For example, compare the Gravity Disturbance page in:

• 0.6.0: https://www.fatiando.org/harmonica/v0.6.0/user_guide/gravity_disturbance.html
• dev: https://www.fatiando.org/harmonica/dev/user_guide/gravity_disturbance.html

I suspect the issue might come from a combination of PyGMT and jupyter-sphinx.