fix(test_sfr): update test to be more robust with Matplotlib versions

[mwtoews]

This fixes CI tests failure that previously worked <24 hrs due to a new release of matplotlib.

Generally, isinstance() is more tolerable, and works here for different versions of matplotlib.

[codecov]

Codecov Report

Merging #1717 (e238d2c) into develop (649596d) will increase coverage by 41.8%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #1717      +/-   ##
==========================================
+ Coverage     30.1%   71.9%   +41.8%     
==========================================
  Files          253     253              
  Lines        54396   55896    +1500     
==========================================
+ Hits         16387   40208   +23821     
+ Misses       38009   15688   -22321     

Impacted Files	Coverage Δ
flopy/mf6/modflow/mfgwtgwt.py	63.0% <0.0%> (-37.0%)	⬇️
flopy/mf6/modflow/mfgwfcsub.py	67.2% <0.0%> (-32.8%)	⬇️
flopy/mf6/modflow/mfgwtdisv.py	68.0% <0.0%> (-32.0%)	⬇️
flopy/mf6/modflow/mfgwtdisu.py	68.4% <0.0%> (-31.6%)	⬇️
flopy/mf6/modflow/mfgwtcnc.py	69.5% <0.0%> (-30.5%)	⬇️
flopy/mf6/modflow/mfgwtsrc.py	69.5% <0.0%> (-30.5%)	⬇️
flopy/mf6/modflow/mfgwfapi.py	69.6% <0.0%> (-30.4%)	⬇️
flopy/mf6/modflow/mfgwfvsc.py	70.2% <0.0%> (-29.8%)	⬇️
flopy/mf6/modflow/mfgwtlkt.py	70.3% <0.0%> (-29.7%)	⬇️
flopy/mf6/modflow/mfgwtmwt.py	70.3% <0.0%> (-29.7%)	⬇️
... and 207 more