Update doctests that were not being run until a recent pytest bugfix

[namurphy]

This PR fixes a few doctests that just started failing.

One thing that's strange... The doctests are now failing both for the newly pinned dependencies as well as the minimal dependencies. This should not be the case.

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[namurphy]

Okay, the reason why it started failing in all of the above is because pytest was skipping doctests if something was decorated with @cached_property (pytest-dev/pytest#11237). That bug just got fixed in the newest release of pytest.

At times like these, I'm glad I spent a couple hours a few years ago setting up an RSS feed of PyPI releases for pretty much every package that PlasmaPy depends on...yeesh. Being able to see the changes in requirements.txt is pretty handy too.

[namurphy]

The only substantive change to this file is rewording this comment. Otherwise, I was just moving things around.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6e63ba4) 98.35% compared to head (82b66e3) 98.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2332   +/-   ##
=======================================
  Coverage   98.35%   98.35%           
=======================================
  Files         100      100           
  Lines        8567     8567           
=======================================
  Hits         8426     8426           
  Misses        141      141           

Files Changed	Coverage Δ
plasmapy/formulary/collisions/frequencies.py	100.00% <ø> (ø)

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[ejohnson-96]

👍