Run v8 python script in a separate subshell

[dylanahsmith]

Problem

build/deps.py was trying to import a script from v8, in a way that isn't python3 compatible, since there aren't init.py files in the directories on the path to the script

Solution

Use a separate command to execute the script, which is the way this is done in v8 as well (https://github.com/v8/v8/blob/ee74e718339223345b42935e9f4cbc7be208e557/DEPS#L552-L558).

[codecov]

Codecov Report

Merging #228 (a13e254) into master (d8d94c2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #228   +/-   ##
=======================================
  Coverage   95.86%   95.86%           
=======================================
  Files          17       17           
  Lines         580      580           
=======================================
  Hits          556      556           
  Misses         15       15           
  Partials        9        9           

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