feat: Add contextlib support to pyhf.schema API

[kratsg]

Pull Request Description

Resolves #1815, originally asked on #1753.

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

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For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add contextlib support to the pyhf.schema API.
   - Follow up to PR #1753.
* Add tests for use of the schema contextlib to tests/test_schema.py.
* Add tests for use of the schema contextlib to the public API tests.

[codecov]

Codecov Report

Merging #1818 (d111ca7) into master (6d03b9e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1818   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files          68       68           
  Lines        4310     4315    +5     
  Branches      725      725           
=======================================
+ Hits         4231     4236    +5     
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.44% <33.33%> (+0.01%)	⬆️
doctest	60.78% <100.00%> (+0.04%)	⬆️
unittests-3.10	96.03% <100.00%> (+<0.01%)	⬆️
unittests-3.7	96.02% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/schema/__init__.py	100.00% <100.00%> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--1818.org.readthedocs.build/en/1818/_generated/pyhf.schema.Schema.html

[matthewfeickert]

Thanks for this PR following up on your nice PR #1753 @kratsg. 🙂 You made this easy to review, so thanks for splitting it out. 👍

I have one super tiny suggestion for the examples, but this might be a nit, and so feel free to ignore. Other than that this LGTM, so I'll approve.

[kratsg]

All resolved.