Cleanup of requirements.txt

[krzywon]

Description

As noted in #2854, some packages are listed multiple times in requirements.txt. This removes those duplicates (sphinx and html5lib) and alphabetizes the list to eliminate the issue in the future.

Note - PySide6 is currently pinned, but that pinning is related to an issue with pyinstaller v6.0+. When #2854 is merged, this pin could potentially be removed (but not for v6.0.0...).

How Has This Been Tested?

Locally created a new python environment using the new file and was able to launch SasView.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

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)

Licencing (untick if necessary)

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

[butlerpd]

I double checked that no requirements were lost (save duplicates) or added. All looks good. However, I cannot test the installer to make sure it is running due to the PyInstaller error. Thus I am merging 6.0 back into branch. Once complete we should test the Win and Mac installer before merging

[butlerpd]

it loads now and runs in Windows. There is a problem with the sector slicer but that is unlikely to be from this pull request. It would be good to get a Mac perspective as well.

[wpotrzebowski]

Tested on Mac.