ci(backport): Use uv for all pip installs

[matthewfeickert]

Description

• Backport:
  • PR ci: Use uv for all pip installs #2444
  • PR ci: Use uv for install on ReadTheDocs #2475

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Backport:
   - PR https://github.com/scikit-hep/pyhf/pull/2444
   - PR https://github.com/scikit-hep/pyhf/pull/2475

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release/v0.7.x@290a23c). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2494   +/-   ##
=================================================
  Coverage                  ?   97.97%           
=================================================
  Files                     ?       69           
  Lines                     ?     4545           
  Branches                  ?      803           
=================================================
  Hits                      ?     4453           
  Misses                    ?       56           
  Partials                  ?       36           

Flag	Coverage Δ
contrib	27.56% <ø> (?)
unittests-3.12	96.25% <ø> (?)
unittests-3.7	96.23% <ø> (?)

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