Work around pip bug

[int-y1]

Fixes #974. I borrowed code from this commit.

[dmoj-build]

Can one of the admins verify this patch?

[codecov-commenter]

Codecov Report

Merging #979 (8088f27) into master (8877809) will increase coverage by 2.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
+ Coverage   80.93%   83.74%   +2.81%     
==========================================
  Files         138      138              
  Lines        4695     4695              
==========================================
+ Hits         3800     3932     +132     
+ Misses        895      763     -132     

Impacted Files	Coverage Δ
dmoj/result.py	82.66% <0.00%> (-1.34%)	⬇️
dmoj/judge.py	52.83% <0.00%> (+1.25%)	⬆️
dmoj/executors/java_executor.py	85.35% <0.00%> (+2.02%)	⬆️
dmoj/executors/compiled_executor.py	77.71% <0.00%> (+4.00%)	⬆️
dmoj/cptbox/tracer.py	77.33% <0.00%> (+16.54%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+25.00%)	⬆️
dmoj/cptbox/isolate.py	86.55% <0.00%> (+47.05%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8877809...8088f27. Read the comment docs.

[kiritofeng]

ok to test