ci: Report coverage for oldest and newest Python tested

[matthewfeickert]

Description

Resolves #1816

• Report coverage for the oldest Python (3.7) and newest (3.10) that are tested to ensure that version specific 'if' statements are properly covered.
• Add Python version specific flags to the coverage reporting to compare differences in coverage and track coverage changes across versions more easily.
• Follow up to PR feat: Alternative Schema Locations #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

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Report coverage for the oldest Python (3.7) and newest (3.10) that are
tested to ensure that version specific 'if' statements are properly covered.
* Add Python version specific flags to the coverage reporting to compare
differences in coverage and track coverage changes across versions more easily.
* Follow up to PR #1753

[codecov]

Codecov Report

Merging #1817 (83a7d1f) into master (2789d54) will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1817      +/-   ##
==========================================
+ Coverage   98.02%   98.16%   +0.13%     
==========================================
  Files          68       68              
  Lines        4310     4310              
  Branches      725      725              
==========================================
+ Hits         4225     4231       +6     
+ Misses         49       46       -3     
+ Partials       36       33       -3     

Flag	Coverage Δ
contrib	26.42% <ø> (ø)
doctest	60.74% <ø> (ø)
unittests	?
unittests-3.10	96.03% <ø> (?)
unittests-3.7	96.01% <ø> (?)

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

Impacted Files	Coverage Δ
src/pyhf/utils.py	94.73% <0.00%> (+5.26%)	⬆️
src/pyhf/schema/loader.py	100.00% <0.00%> (+9.52%)	⬆️
src/pyhf/schema/variables.py	100.00% <0.00%> (+25.00%)	⬆️

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[kratsg]

I see flags. This looks good