Fixs for pint problem in docs build action

[jacobcook1995]

Description

It seems like pint have changed how they do typing, which caused documentation build failures. I have set intersphinx to read from an earlier version of the pint documentation to avoid this problem.

This will have to change in future if we update pint, but at the moment it seems like they are still trying to pin down their typing, so doesn't seem sensible to refactor the code at the moment.

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov-commenter]

Codecov Report

Merging #218 (fa3b01d) into develop (a3a1800) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #218   +/-   ##
========================================
  Coverage    94.95%   94.95%           
========================================
  Files           33       33           
  Lines         1408     1408           
========================================
  Hits          1337     1337           
  Misses          71       71           

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[vgro]

LGTM :-)