py-pexpect: updated to 4.3.0

Version 4.3
The async= parameter to integrate with asyncio has become async_= (:ghpull:`431`), as async is becoming a Python keyword from Python 3.6. Pexpect will still recognise async as an alternative spelling.
Similarly, the module pexpect.async became pexpect._async (:ghpull:`450`). This module is not part of the public API.
Fix problems with asyncio objects closing file descriptors during garbage collection (:ghissue:`347`, :ghpull:`376`).
Set the .pid attribute of a :class:`~.PopenSpawn` object (:ghpull:`417`).
Fix passing Windows paths to :class:`~.PopenSpawn` (:ghpull:`446`).
:class:`~.PopenSpawn` on Windows can pass string commands through to Popen without splitting them into a list (:ghpull:`447`).
Stop shlex trying to read from stdin when :class:`~.PopenSpawn` is passed cmd=None (:ghissue:`433`, :ghpull:`434`).
Ensure that an error closing a Pexpect spawn object raises a Pexpect error, rather than a Ptyprocess error (:ghissue:`383`, :ghpull:`386`).
Cleaned up invalid backslash escape sequences in strings (:ghpull:`430`, :ghpull:`445`).
The pattern for a password prompt in :mod:`pexpect.pxssh` changed from password to password: (:ghpull:`452`).
Correct docstring for using unicode with spawn (:ghpull:`395`).
Various other improvements to documentation.


(adam)