ci: Add CPython 3.10 to testing

[matthewfeickert]

Description

Resolves #1758

Now that PyTorch v1.11.0 is out with Python 3.10 support, we can add CPython 3.10 to the CI. A follow up PR will add Python 3.10 to the metadata.

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

* Add CPython 3.10 to the testing matrix in CI.
* Use CPython 3.10 as the default Python for all CI/CD workflows.

[codecov]

Codecov Report

Merging #1808 (d1e0db1) into master (47ae6ad) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1808      +/-   ##
==========================================
+ Coverage   98.12%   98.14%   +0.02%     
==========================================
  Files          64       64              
  Lines        4270     4270              
  Branches      683      719      +36     
==========================================
+ Hits         4190     4191       +1     
  Misses         46       46              
+ Partials       34       33       -1     

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.60% <ø> (+0.02%)	⬆️
unittests	96.13% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
src/pyhf/optimize/__init__.py	100.00% <0.00%> (+4.54%)	⬆️

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[matthewfeickert]

I'm going to merge this as adding additional CI testing probably isn't controversial, but I'm happy to have any follow up discussions or revert things as needed.