Update the intersphinx urls as the warnings suggested

[peterfpeterson]

Building the docs-html target was warning about needing to update intersphinx links. This does what the warnings said to do.

There is no associated issue.

To test:

ninja docs-html

before and after this. One will show intersphinx URL warnings. Hopefully, it is the first one.

This does not require release notes because it is fixing a build server warning.

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[backmari]

I ran ninja docs-html before and after these changes.

Before:

Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`.
loading intersphinx inventory 'h5py' from https://h5py.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory 'matplotlib' from https://matplotlib.org/objects.inv...
loading intersphinx inventory 'numpy' from https://numpy.org/doc/stable/objects.inv...
loading intersphinx inventory 'python' from https://docs.python.org/3/objects.inv...
loading intersphinx inventory 'SciPy' from https://docs.scipy.org/doc/scipy/reference/objects.inv...
loading intersphinx inventory 'pandas' from https://pandas.pydata.org/pandas-docs/stable/objects.inv...
loading intersphinx inventory 'pystog' from https://pystog.readthedocs.io/en/latest/objects.inv...
loading intersphinx inventory 'mantid-dev' from https://developer.mantidproject.org/objects.inv...
intersphinx inventory has moved: https://matplotlib.org/objects.inv -> https://matplotlib.org/stable/objects.inv
intersphinx inventory has moved: https://docs.scipy.org/doc/scipy/reference/objects.inv -> https://docs.scipy.org/doc/scipy/objects.inv
intersphinx inventory has moved: https://h5py.readthedocs.io/en/stable/objects.inv -> https://docs.h5py.org/en/stable/objects.inv
building [mo]: targets for 0 po files that are out of date

After (the three "has moved" messages are gone):

Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`.
loading intersphinx inventory 'h5py' from https://docs.h5py.org/en/stable/objects.inv...
loading intersphinx inventory 'matplotlib' from https://matplotlib.org/stable/objects.inv...
loading intersphinx inventory 'numpy' from https://numpy.org/doc/stable/objects.inv...
loading intersphinx inventory 'python' from https://docs.python.org/3/objects.inv...
loading intersphinx inventory 'SciPy' from https://docs.scipy.org/doc/scipy/objects.inv...
loading intersphinx inventory 'pandas' from https://pandas.pydata.org/pandas-docs/stable/objects.inv...
loading intersphinx inventory 'pystog' from https://pystog.readthedocs.io/en/latest/objects.inv...
loading intersphinx inventory 'mantid-dev' from https://developer.mantidproject.org/objects.inv...
building [mo]: targets for 0 po files that are out of date

[backmari]

This change makes the docs build warnings go away as described.