Fixed in-place editing of dir list while iterating

[akx]

This is a reissue of #476, since Git and Github and Travis were being silly.

[akx]

Thanks @MarcDufresne :)

[codecov-io]

Codecov Report

Merging #492 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
+ Coverage   90.17%   90.19%   +0.02%     
==========================================
  Files          24       24              
  Lines        3979     3977       -2     
==========================================
- Hits         3588     3587       -1     
+ Misses        391      390       -1

Impacted Files	Coverage Δ
babel/messages/extract.py	95.27% <100%> (+0.32%)	⬆️

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