Fix ModuleNotFoundError error when IPython is not installed (#1247)
Fix ModuleNotFoundError error when IPython is not installed (#1247)

* Fix the exception when IPython is not installed
* Install IPython on the Python 3.9/NumPy 1.20 test build only
* Revert "Install IPython on the Python 3.9/NumPy 1.20 test build only"

This reverts commit db3641051ca5e4a13c1baedce9b07a7ca096f589.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python