Switch scour over to python3 now that we have Inkscape 1.0
Summary: Thanks to Inkscape 1.0 now being in the repos, we can finally drop scour from python2 and move it to python3 :)
Test Plan: - Save the same Inkscape SVG optimized and unoptimized to make sure that the optimized actually is smaller.
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Differential Revision: https://dev.getsol.us/D8848