d/b/genpatch-rt: Fix subprocess cleanup with Python 3.13

Quoting Colin Watson from wireless-regdb@c464cfbb:

With Python 3.13, reopening the stdout file descriptor [...] causes cleanup to fail later. It's simpler to call subprocess.Popen with text=True so that we get text output directly

Since genpatch-rt is used during source package preparation, this needs to be fixed on older branches, not just those where the target release has Python 3.13+.