Write in Python a function \texttt{moyenne\_tours(p, n, nbe)} that takes as arguments a probability $p$, the order $n$ of the matrices $A _ { k }$ and an integer \texttt{nbe} and that returns the average, over \texttt{nbe} trials performed, of the number of steps necessary to go from $A _ { 0 }$ to $- A _ { 0 }$.