bpo-36673: Implement comment/PI parsing support for the TreeBuilder in ElementTree. (#12883)

* bpo-36673: Implement comment/PI parsing support for the TreeBuilder in ElementTree.

* bpo-36673: Rewrite the comment/PI factory handling for the TreeBuilder in "_elementtree" to make it use the same factories as the ElementTree module, and to make it explicit when the comments/PIs are inserted into the tree and when they are not (which is the default).