BTW, when you will check point 3 you will notice that we already read id output, so command execute correctly: "uid=0(root" ...
And problem is after that.

Steps I reproduce this issue:
- standard user have keys and
 in ~/.ssh/config
Host *
   ControlMaster auto
   ControlPath /tmp/ssh-%r@%h:%p
   ControlPersist yes

- on root (authorized_key installed)
cat ~user/.ssh/id_rsa.pub >> ~root/.ssh/authorized_keys

- on user, reproduce
ssh root@localhost -O exit # destroy mux
strace python test.py # script I paste before

I hit this problem of few machines, with different linux kernels ...
Seems like generic problem?