use a standard function to check for iterable collections

use a standard function to check for iterable collections

Fixed 2.0 regression in :class:`.MutableList` where a routine that detects
sequences would not correctly filter out string or bytes instances, making
it impossible to assign a string value to a specific index (while
non-sequence values would work fine).

Fixes: #10784
Change-Id: I829cd2a1ef555184de8e6a752f39df65f69f6943