Update matplotlib supported versions

[nfahlgren]

Describe your changes
Removes upper limit on matplotlib versions and fixes one issue in Points related to changes to the matplotlib API. The change is backwards compatible with older versions of matplotlib.

Type of update
Is this a: update

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #1353 (c4a8ab8) into main (ebd9b96) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1353   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          155       155           
  Lines         6841      6840    -1     
=========================================
- Hits          6841      6840    -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
plantcv/plantcv/classes.py	100.00% <100.00%> (ø)