Revert "Port Checker to python 3"

This reverts commit 961f2954c12cfd66970a3150f2e3ef9b2567300f.

Reason for revert: Breaks some Checker tests on the ART Buildbot (see b/170308859).

Bug: 170308859
Bug: 162408889
Change-Id: If531b015c2aa23d4e64e9e349982dc72cf8dc093
10 files changed