Update pexpect to 4.3.1

[pyup-bot]

There's a new version of pexpect available.
You are currently using 4.2.1. I have updated it to 4.3.1

These links might come in handy: PyPI | Changelog | Docs

Changelog

4.3.1

• When launching bash for :mod:pexpect.replwrap, load the system bashrc
  from a couple of different common locations (:ghpull:457), and then unset
  the PROMPT_COMMAND environment variable, which can interfere with the
  prompt we're expecting (:ghpull:459).

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.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #201