[Checkins] [zopefoundation/zope.testrunner] aa83fa: Avoid PowerShell in Appveyor

[Marius Gedminas]

  Branch: refs/heads/py38
  Home:   https://github.com/zopefoundation/zope.testrunner
  Commit: aa83fad63ee3b236a32cf71df7379c6f1171234e
      https://github.com/zopefoundation/zope.testrunner/commit/aa83fad63ee3b236a32cf71df7379c6f1171234e
  Author: Marius Gedminas <marius at gedmin.as>
  Date:   2019-02-12 (Tue, 12 Feb 2019)

  Changed paths:
    M appveyor.yml

  Log Message:
  -----------
  Avoid PowerShell in Appveyor

PowerShell treats any output to standard error (such as pip's
deprecation warning about Python 2.7 and 3.4 reaching EOL soon) as a
fatal error.