remove trailing slash from base URLs on readthedocs.io

[drammock]

on a recent MNE-Python doc build I see that trailing slashes in the base URL are causing redirection warnings:

intersphinx inventory has moved: https://pybv.readthedocs.io/en/latest//objects.inv -> https://pybv.readthedocs.io/en/latest/objects.inv

intersphinx inventory has moved: https://imageio.readthedocs.io/en/stable//objects.inv -> https://imageio.readthedocs.io/en/stable/objects.inv

intersphinx inventory has moved: https://joblib.readthedocs.io/en/latest//objects.inv -> https://joblib.readthedocs.io/en/latest/objects.inv

intersphinx inventory has moved: https://numba.readthedocs.io/en/latest//objects.inv -> https://numba.readthedocs.io/en/latest/objects.inv

intersphinx inventory has moved: https://patsy.readthedocs.io/en/latest//objects.inv -> https://patsy.readthedocs.io/en/latest/objects.inv

intersphinx inventory has moved: https://pyqtgraph.readthedocs.io/en/latest//objects.inv -> https://pyqtgraph.readthedocs.io/en/latest/objects.inv

I'm assuming it affects all inventories stored on readthedocs.io so I've changed all of them, not just the ones MNE uses. Should reduce noise in everyone's logs.

[Carreau]

thanks.