test: Temporarily move to testing against SciPy pre-releases

[matthewfeickert]

Description

As described in Issue #1531, the nightly tests against the HEAD of SciPy's source have been failing for so long that they are no longer useful and are generating too many false positives (test failures) for the HEAD of dependencies workflow to be a useful test. Until the build issue can be resolved, replace testing from HEAD against testing against releases with release candidates as eligible installs.

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

* Temporarily remove installs of SciPy from the HEAD of source to reduce false failures in 'HEAD of dependencies' workflow
   - c.f. Issue #1531
* Use pip install --pre to test against release candidates of SciPy

[codecov]

Codecov Report

Merging #1532 (653c9cd) into master (025ffe9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1532   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          63       63           
  Lines        4006     4006           
  Branches      565      565           
=======================================
  Hits         3912     3912           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.51% <ø> (ø)
unittests	97.42% <ø> (ø)

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