Go back to using subprocess.Popen for :restart

With subprocess.run, we wait until the subprocess has completed, which means the
parent process will hang on :restart.

Since we *don't* want to wait for the subprocess here, using subprocess.Popen
seems the right thing to do.

This was introduced in bb54a954fe / #3203
Probably doesn't affect #3210 and #3220.

qutebrowser/app.py

@@ -660,9 +660,9 @@ class Quitter:
         try:
             args, cwd = self._get_restart_args(pages, session, override_args)
             if cwd is None:
-                subprocess.run(args)
+                subprocess.Popen(args)
             else:
-                subprocess.run(args, cwd=cwd)
+                subprocess.Popen(args, cwd=cwd)
         except OSError:
             log.destroy.exception("Failed to restart")
             return False