launcher: support Python 3.5 for now
launcher: support Python 3.5 for now

The codebase still supports Python 3.5, so allow use of that instead
of requiring Python 3.6+.  Supporting this mode well is a bit tricky
as we want to first scan for newer versions before falling back to
older ones.  And we have to avoid infinite loops in the process.

Bug: https://crbug.com/gerrit/13795
Change-Id: I47949a173899bfa9ab20d3fefa1a97bf002659f6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/292442
Reviewed-by: Michael Mortensen <668d7528ffdc223b4950b555aea5a48377d73cb0@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

lipro-yocto/git-repo,GerritCodeReview/git-repo,GerritCodeReview/git-repo,lipro-yocto/git-repo,lewixliu/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,couchbasedeps/git-repo