Write in Python a function count$\_$letterthat takes a list of strings and a string letter as parameters and returns the number of times this letter occurs, both upper$-$ & lower$-$cased.

Example function call:

list $=[$HELLO$,$goodbye$,1234$ Oooh!$]$

print$($count$\_$letter$($list$,$o$))$

Outputs: $6$