Fix sas.models suppression to not accidentally allow private members into sphinx docs

[llimeht]

Description

Previous changes had allowed private names (variables, functions, methods starting with _ or __) to leak into the sphinx API docs. The result was that they were full of lots of meaningless items that should be hidden from the user. This patch restores the name (in)visibility in the API docs.

How Has This Been Tested?

The file that initially drew this to our attention was dev/sasview-api/sas.qtgui.Perspectives.ParticleEditor.datamodel.html - it came up because the private functions that were included into the docs had irreproducible signatures.

As seen in the CI outputs here, that file no longer contains these private names. This also makes the package reproducible, an important step in software assurance.

This patch also removes from very very old compatibility code for Sphinx 2.x.

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)