Drop use of six.u

[pganssle]

Since we no longer support Python 2.6, there's really no reason to use six.u anymore, but it is still used in easy_install:

tmpdir = tempfile.mkdtemp(prefix=six.u("easy_install-"))

This can be replaced with a u-prefixed string until we're Python 3-only:

tmpdir = tempfile.mkdtemp(prefix=u"easy_install-")