depot_tools codebase uses py3 and there's limited py2 usage, namely for
presubmits. This CL drops running most of py2 unit tests since there are
no longer useful.

Bug: 1357152
Change-Id: I49a3df503026bc6918362a9b5600f5714eb5c87a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3872425
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>