executors: fix returning str instead of bytes from get_compile_output

[Xyene]

No description provided.

[codecov-commenter]

Codecov Report

Merging #960 (767b502) into master (08e69d7) will decrease coverage by 0.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #960      +/-   ##
==========================================
- Coverage   86.54%   85.98%   -0.57%     
==========================================
  Files         138      138              
  Lines        4735     4730       -5     
==========================================
- Hits         4098     4067      -31     
- Misses        637      663      +26     

Impacted Files	Coverage Δ
dmoj/executors/CBL.py	95.23% <100.00%> (ø)
dmoj/executors/PAS.py	100.00% <100.00%> (ø)
dmoj/executors/TUR.py	69.23% <0.00%> (-19.24%)	⬇️
dmoj/executors/SWIFT.py	90.90% <0.00%> (-9.10%)	⬇️
dmoj/executors/D.py	91.66% <0.00%> (-8.34%)	⬇️
dmoj/executors/asm_executor.py	77.63% <0.00%> (-5.93%)	⬇️
dmoj/cptbox/tracer.py	82.90% <0.00%> (-2.19%)	⬇️
dmoj/result.py	89.33% <0.00%> (-1.34%)	⬇️
dmoj/executors/compiled_executor.py	88.06% <0.00%> (-1.14%)	⬇️
dmoj/cptbox/isolate.py	94.95% <0.00%> (-0.85%)	⬇️
... and 3 more

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