Work around changes to argparse private API

[cottsay]

Unfortunately, achieving colcon's arbitrary argument parsing involves reaching into argparse private API. There was recently a change to that API which was backported to stable Python releases, and this change is necessary to restore the desired behavior.

[cottsay]

CI is still broken due to colcon/ci#34, which could also use a review.

[clalancette]

I'm completely fine with this as a change to unbreak colcon, so approving.

But at this point, it might be worth considering contributing to upstream argparse to add in some kind of functionality so we are using a "stable" API.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.37%. Comparing base (faee881) to head (b3154c3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   87.32%   87.37%   +0.05%     
==========================================
  Files          68       68              
  Lines        3945     3945              
  Branches      777      756      -21     
==========================================
+ Hits         3445     3447       +2     
  Misses        396      396              
+ Partials      104      102       -2     

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