Update CHANGES
Remove misleading ref to 5.0 release from HISTORY
Change version to 4.3

The last released version was 4.2. There almost was a 5.0, with
SIGCHLD handler changes in it, but it never saw the light of
day. Since we don't have major changes so far, the next
version should tentatively be called "4.3".
Add support for pickling floats
Merge branch 'master' of https://github.com/ChrisJefferson/io