[issue21647] Idle unittests: make gui, mock switching easier.

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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assignee:  -> terry.reedy
components: +IDLE
versions: +Python 3.7, Python 3.8 -Python 2.7, Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21647>
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