fix(mf6): allow using existing dfn files in generate_classes.py

[wpbonelli]

Previously failed with shutil.SameFileError if you tried to regenerate classes using the existing set of DFNs e.g.

python -m flopy.mf6.utils.generate_classes --dfnpath flopy/mf6/data/dfn --no-backup

Now it will work.

[codecov]

Codecov Report

Attention: Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.

Project coverage is 76.3%. Comparing base (bb9824e) to head (7582c35).
Report is 43 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/generate_classes.py	9.0%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2431     +/-   ##
=========================================
+ Coverage     68.4%   76.3%   +7.8%     
=========================================
  Files          294     294             
  Lines        59390   59934    +544     
=========================================
+ Hits         40652   45739   +5087     
+ Misses       18738   14195   -4543     

Files with missing lines	Coverage Δ
flopy/mf6/utils/generate_classes.py	17.8% <9.0%> (+<0.1%)	⬆️

... and 247 files with indirect coverage changes