subunit/run.py: change shebang to python3

always use explicit python version at current stage.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Noel Power <npower@samba.org>

Autobuild-User(master): Noel Power <npower@samba.org>
Autobuild-Date(master): Thu Mar  7 13:03:56 UTC 2019 on sn-devel-144
