[issue26819] _ProactorReadPipeTransport pause_reading()/resume_reading() broken if called before any read is perfored

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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versions: +Python 3.6, Python 3.7, Python 3.8 -Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue26819>
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