Issue #20643: Fixed references to the next() method (distinguish from the
next() function).
Update OS X installer build script for changes to documentation build:
as of 2.7.9, doc builds require an externally installed sphinx-build
like 3.4+ builds do.
Document 2.7.9 changes in OS X installer build requirements:
because the Doc Makefile has been changed to no longer download sphinx
and its dependencies, build-installer.py now requires that there be an
externally-supplied sphinx-build available.