Test build to update docs to sphinx 4.x

[ascillitoe]

This is a draft PR to update the docs to use sphinx>=4.0. There is currently a problem documenting classes that inherit mocked classes (e.g. alibi_detect.utils.pytorch.kernels.DeepKernel), which is sensitive to the sphinx version:

• sphinx<3.0: Everything is OK here.
• sphinx>=3.0 < 3.4.2: Classes that inherit mocked classes are not documented. (see sphinx #8164). This is fixed in sphinx 3.4.2.
• sphinx>=3.4.2: The previously missing classes are now documented, but there is a problem with the "Bases" section in the docs. Example: In alibi_detect.utils.pytorch.kernels.DeepKernel "Bases: torch.nn." instead of "Bases: torch.nn.Module".

See here for an example readthedocs build.

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[ascillitoe]

Screenshot with sphinx==4.2

Screenshot with sphinx<3.0

[ascillitoe]

Update: looks like a fix for this might be on its way in sphinx 4.3.0, due approx. early November. Hopefully, we can go ahead and update the docs at that point. See sphinx-doc/sphinx#9658.

[ascillitoe]

Looks like sphinx v4.3.0 might take a little longer than expected to be released. Merging this now so that we can build docs for the new non-parametric detectors (#359). I will leave SeldonIO/alibi#505 open as a reminder for us to check the issue with Bases is fixed by v4.3.0 when it is released.