fix(mflist_export): patch mf6 list data export

[wpbonelli]

Patch #2531. Exporting stress_period_data still breaks if use_pandas=False but since the default is True and I know of no use case to set it False I am going to leave this fractally expanding terrain for followup work

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.5%. Comparing base (556c088) to head (090b54d).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #2532    +/-   ##
=========================================
  Coverage     55.5%    55.5%            
=========================================
  Files          644      642     -2     
  Lines       124135   124242   +107     
=========================================
+ Hits         68947    69047   +100     
- Misses       55188    55195     +7     

Files with missing lines	Coverage Δ
flopy/export/utils.py	64.8% <100.0%> (+0.2%)	⬆️

... and 39 files with indirect coverage changes

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