[MRG] fix ResourceWarning from unclosed file

[ctb]

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality, is it tested?
• make format diff_pylint_report doc Is it well formatted?
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Base: 87.51% // Head: 87.53% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (b50683e) compared to base (e2232ae).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest      #90      +/-   ##
==========================================
+ Coverage   87.51%   87.53%   +0.01%     
==========================================
  Files          15       15              
  Lines         633      634       +1     
==========================================
+ Hits          554      555       +1     
  Misses         79       79              

Impacted Files	Coverage Δ
screed/openscreed.py	92.51% <100.00%> (+0.04%)	⬆️

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