Use astor master to see if we fix the python3.5 compat

[stefanoborini]

Python 3.5 generates an error in astor 0.5. We replace it with master and see if it works.

[codecov-io]

Codecov Report

Merging #494 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #494   +/-   ##
=======================================
  Coverage   95.24%   95.24%           
=======================================
  Files          88       88           
  Lines        3952     3952           
  Branches      247      247           
=======================================
  Hits         3764     3764           
  Misses        137      137           
  Partials       51       51

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