[issue38756] Add generic versions of weakref types to typing module

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan <tir.karthi at gmail.com>:


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nosy: +gvanrossum, levkivskyi

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38756>
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