remove six dependency install

[edgarriba]

pytorch/pytorch#27282 seems to be merged

[codecov-io]

Codecov Report

Merging #1792 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1792   +/-   ##
======================================
  Coverage    0.28%   0.28%           
======================================
  Files          92      92           
  Lines        7360    7360           
  Branches     1110    1110           
======================================
  Hits           21      21           
  Misses       7331    7331           
  Partials        8       8

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[fmassa]

Thanks!