Marker bugfix

[CSSFrancis]

Description of the change

These are just some small bugs I've found downstream in a couple of places.

See #3269 for more details.

Progress of the PR

• Change implemented (can be split into several points),
• add example
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

print(hs.plot.markers.Points.from_signal(s))

(see the example for a more detailed example

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f75a8d7) 80.86% compared to head (206b6e4) 80.86%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3270      +/-   ##
======================================================
- Coverage               80.86%   80.86%   -0.01%     
======================================================
  Files                     140      140              
  Lines                   20400    20399       -1     
  Branches                 4826     4825       -1     
======================================================
- Hits                    16496    16495       -1     
- Misses                   2827     2829       +2     
+ Partials                 1077     1075       -2     

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[ericpre]

I open #3274 to fix the test suite. Once #3274 is merged, we can rebase this PR.