docs: document a Python 3 migration plan
docs: document a Python 3 migration plan

Bug: https://crbug.com/gerrit/10418
Change-Id: I72d82ce3a2d9af45d942bb10de82340110864ea5

lewixliu/git-repo,aosp-mirror/tools_repo,codetutil/git-repo,couchbasedeps/git-repo,lipro-yocto/git-repo,gabbayo/git-repo,lipro-yocto/git-repo,TheQtCompany/git-repo,mer-tools/git-repo,chenzilin/git-repo,aosp-mirror/tools_repo,GerritCodeReview/git-repo,GerritCodeReview/git-repo,gabbayo/git-repo,mer-tools/git-repo,TheQtCompany/git-repo,codetutil/git-repo,couchbasedeps/git-repo,chenzilin/git-repo,lewixliu/git-repo