fix(backport): Remove os-dependent pieces of schema validation

[matthewfeickert]

Description

• Backport PR fix: Remove os-dependent pieces of schema validation #2357
• Use pathlib to build the stem for the schema to use (version + type of schema).
  • c.f. pathlib strips trailing slash python/cpython#65238

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/2357
* Use pathlib to build the stem for the schema to use (version + type of schema).
   - c.f. https://github.com/python/cpython/issues/65238

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (release/v0.7.x@9caaf1d). Click here to learn what that means.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2361   +/-   ##
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  Coverage                  ?   98.21%           
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  Files                     ?       69           
  Lines                     ?     4543           
  Branches                  ?      803           
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  Hits                      ?     4462           
  Misses                    ?       47           
  Partials                  ?       34           

Flag	Coverage Δ
contrib	27.53% <0.00%> (?)
doctest	60.99% <0.00%> (?)
unittests-3.10	96.25% <0.00%> (?)
unittests-3.11	96.25% <0.00%> (?)
unittests-3.7	96.23% <0.00%> (?)
unittests-3.8	96.27% <0.00%> (?)
unittests-3.9	96.30% <0.00%> (?)

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