flush Python I/O in IJulia

[stevengj]

Fixes #912.

[codecov-commenter]

Codecov Report

Merging #982 (c70f8b5) into master (6b18f38) will increase coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #982      +/-   ##
==========================================
+ Coverage   68.33%   68.36%   +0.03%     
==========================================
  Files          20       20              
  Lines        2021     2023       +2     
==========================================
+ Hits         1381     1383       +2     
  Misses        640      640              

Flag	Coverage Δ
unittests	68.36% <50.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyinit.jl	83.00% <50.00%> (-0.68%)	⬇️
src/conversions.jl	63.56% <0.00%> (+0.24%)	⬆️

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