Migrate build to python 3

Update the python version required in local scripts and in bazlets.

bazlets log:
$ git log --no-merges --oneline ^a52e3f381 00a97f961
b2b8548 download to GERRIT_CACHE_HOME when set
2d0cc7e Migrate to python 3
6311520 tools/maven/package.bzl: Use bazelisk instead bazel

Change-Id: I7a0671a0971ac91257ebca0aee8f8710dd5544a1