run_tests: rewrite to use Python 3
run_tests: rewrite to use Python 3

Some distros still have `pytest` as Python 2 and sep `pytest-3`.
Rewrite this script to use `pytest-3` if available.

Change-Id: I430ed8792e7b0da9b217f948f2e983aa62bf1299
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/290503
Reviewed-by: Michael Mortensen <668d7528ffdc223b4950b555aea5a48377d73cb0@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

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