How to use an associative array in post__in with WP_Query?

Take a look at PHP’s array_values function. Alternatively, you could typecast.

Note that if the array is in serialized form as you have put it above, you will have to unserialize it first, in either case.

$numerical_array = array_values(
    unserialize(
        'a:1:{s:8:"post-134";s:3:"134";s:8:"post-136";s:3:"136";}'
    )
);

or

$numerical_array = (array) unserialize(
        'a:1:{s:8:"post-134";s:3:"134";s:8:"post-136";s:3:"136";}'
);