fix(mfdataplist): fix dangling file handle

[wpbonelli]

#2609 left a file handle open

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (3211553).
⚠️ Report is 54 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2615      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      667      +23     
  Lines       124135   128908    +4773     
===========================================
+ Hits         68947    93556   +24609     
+ Misses       55188    35352   -19836     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdataplist.py	73.8% <100.0%> (-5.0%)	⬇️

... and 553 files with indirect coverage changes

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