Update change log.

Version number -> 4.6

Clarify release notes for 4.5

Fix (hopefully) email address for PyPI upload

Version number -> 4.3.1

Version number -> 4.3

Version 4.2.1

Merge pull request pexpect#361 from pexpect/restore-posix

Prepare 4.2 release

Version 4.1.0

4.0.1 release