merge with 3.4

diff --cc Doc/howto/pyporting.rst
Simple merge

diff --cc Doc/library/collections.rst
Simple merge

diff --cc Doc/library/difflib.rst
Simple merge

diff --cc Doc/library/gettext.rst
Simple merge

diff --cc Doc/library/plistlib.rst
Simple merge

diff --cc Doc/library/select.rst
Simple merge

diff --cc Doc/library/unittest.rst
Simple merge