fix: Ensure pytest.approx only compares floats

[matthewfeickert]

Description

Resolves #1219

In response to Issue #1219, this PR avoids the issues that pytest v6.2.0 brings by restricting pytest to the v6.1.X series.
This PR ensures that there floats are compared instead of floats to tensors. To try to catch these sorts of issues sooner and maybe bring them up with the pytest devs add pytest to the HEAD of dependencies workflow as well.

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

* Compare floats to floats and not tensors in pytest.approx
* Add pytest and uproot3 to HEAD of dependencies workflow
* Restrict sphinxcontrib-bibtex to v1.X
   - c.f. PR #1221

[codecov]

Codecov Report

Merging #1220 (524fc96) into master (f824afe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1220   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files          63       63           
  Lines        3688     3688           
  Branches      524      524           
=======================================
  Hits         3594     3594           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.45% <ø> (ø)

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