[issue34787] imghdr raise TypeError for PNG

[lorenzogotuned]

New submission from lorenzogotuned <tunedconsulting at gmail.com>:

Found on Python 3.6

This line https://github.com/python/cpython/blob/master/Lib/imghdr.py#L45
always raises`TypeError`as `startswith` does not accept bytes.

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messages: 326237
nosy: lorenzogotuned
priority: normal
severity: normal
status: open
title: imghdr raise TypeError for PNG
type: crash

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34787>
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