estimation of ARMA(1,1) with MA parameter greater than 1
I suspect that the reason is that many ARMA fitting algorithms require that the ARMA system be stably invertible.  In your example, the MA process is not minimum phase as the zero will be outside the unit circle. That makes the inverted system unstable (the pole will be outside the unit circle).

I don't know the details of the sm.tsa.ARMA(y, (1, 1)).fit algorithm, but I suspect that it will also be so restricted.

See, for example, this lecture by Florian Pelgrin, which contains the slide below.



In particular, note the second condition.

This means that I suspect it will be harder to find an algorithm that does the estimation for the case you're  interested in.