ci: Add matplotlib nightly wheels to HEAD of dependencies testing

[matthewfeickert]

Description

With the addition of matplotlib nightly wheels on the scipy-wheels-nightly Anaconda Cloud organization package index (c.f. matplotlib/matplotlib#22733), wheels that are either at, or very close to, matplotlib's HEAD on GitHub can now be tested.

As the only place that matplotlib is used in pyhf is in pyhf.contrib, only test the contrib module instead of all of pyhf.

Note for devs:

• Need to use --extra-index-url as dependencies aren't on scipy-wheels-nightly package index.
• Need to use --pre as dev releases will need priority over stable releases.

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

* With the addition of matplotlib nightly wheels on the
scipy-wheels-nightly Anaconda Cloud organization package index,
wheels that are either at, or very close to, matplotlib's HEAD
on GitHub can now be tested. As the only place that matplotlib
is used in pyhf is in pyhf.contrib, only test the contrib module
instead of all of pyhf.
   - c.f. https://anaconda.org/scipy-wheels-nightly

[codecov]

Codecov Report

Merging #1835 (b5917fe) into master (d8158cf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1835   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files          68       68           
  Lines        4315     4315           
  Branches      725      725           
=======================================
  Hits         4236     4236           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.44% <ø> (ø)
doctest	60.78% <ø> (ø)
unittests-3.10	96.03% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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