Issue #17415: Clarify 'this' referent by moving containing sentence just after
the sentence referred to. Make other minor edits to improve flow.
Issue #17415: Trim trailing whitespace
#16057: Clarify why the base method default is called in custom encoders.

Original patch by Kushal Das.
#17448: Make test_sax skip if there are no xml parsers.

Patch by Rafael Santos.