Write a function that takes a username as input and replaces certain characters with digits or special characters that resemble them visually. However, replacements should only be made on the even occurrence of those characters in the username, not the odd occurrences. In other words, the first time you see a character that can be replaced, we won't change it$.$ The second time you see a character that can be replaced, you should, and so on$.$ Note that the input will always be lower$-$cased.

Arguments:

$-$ username $($type: str$)$: The input username.

Returns:

$-$ fancy$\_$username $($type: str$)$: The modified username with replacements made.

Character Mappings:

a $->$ @

e $->3$

i $->1$

o $->0$

s $->$ $

g $->9$

b $->6$

Examples:

create$\_$fancy$\_$username$($username$)$: us$3$rnam$3$

create$\_$fancy$\_$username$($association$)$: a$s$0$ci@ti$0$n

create$\_$fancy$\_$username$($mississippi$)$: mi$s$1$s$ipp$1$

Starting code:

def create$\_$fancy$\_$username$($username: str$)->$ str: