TST: oops, named source file wrong in test suite

[klauer]

• *.txt isn't picked up by the conftest mechanism
• *.st is
• Rename .txt -> .st for it to be covered by the test suite

[codecov-commenter]

Codecov Report

Merging #45 (bb59659) into master (e572219) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   71.68%   71.97%   +0.29%     
==========================================
  Files          18       18              
  Lines        3754     3754              
==========================================
+ Hits         2691     2702      +11     
+ Misses       1063     1052      -11     

Impacted Files	Coverage Δ
blark/summary.py	60.48% <0.00%> (+0.44%)	⬆️
blark/_version.py	39.16% <0.00%> (+2.67%)	⬆️

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