fix(export_contours/f): support matplotlib 3.8+

[wpbonelli]

• close bug: export_contours broken with matplotlib 3.8 #1950
• update tests for easier visual debugging of exported shapefiles
• update export_contours() and export_contourf() for matplotlib 3.8+ compat
• add verbose flag to export_contours() to match export_contourf()

See this comment for context

[codecov]

Codecov Report

Merging #1951 (5a41f5c) into develop (2fa32da) will increase coverage by 0.0%.
Report is 1 commits behind head on develop.
The diff coverage is 92.0%.

@@           Coverage Diff            @@
##           develop   #1951    +/-   ##
========================================
  Coverage     72.6%   72.6%            
========================================
  Files          257     257            
  Lines        57711   57812   +101     
========================================
+ Hits         41927   42020    +93     
- Misses       15784   15792     +8     

Files Changed	Coverage Δ
flopy/export/utils.py	66.4% <92.0%> (+1.7%)	⬆️

... and 1 file with indirect coverage changes

[christianlangevin]

Hey @wpbonelli, this looks good. Too bad about having to handle versions differently, but seems appropriate for now, at least until pre-matplotlib 3.8 is deprecated. Only real comment is the string check on the matplotlib version. We should keep our eyes out for a lightweight generic python package that can create contours from regularly and irregularly spaced data. I poked around but no luck.

[wpbonelli]

@langevin-usgs it looks like MPL uses contourpy, maybe we could drop down to that directly in the longer term?

In the meantime, a patch release seems in order after this merges, for interim support for all MPL versions

[christianlangevin]

@wpbonelli, seems like considering contourpy for this in the future would be worth exploring, but not a high priority. Agree with a patch release. Thanks for fixing.