What did you do?

My library BentoML is using imageio and Pillow in its tests, and with the latest release of Pillow 7.1.0 half an hour ago, I noticed some existing tests are breaking.

https://github.com/bentoml/BentoML
https://travis-ci.org/github/bentoml/BentoML/jobs/669765333?utm_medium=notification&utm_source=github_status

What did you expect to happen?

imageio.imread('./image.png') should work

What actually happened?

AttributeError: 'PngImageFile' object has no attribute '_PngImageFile__frame'

What are your OS, Python and Pillow versions?

• OS: mac os
• Python: 3.75
• Pillow: 7.1.0
• imageio: 2.8.0

from imageio import imread
imread('./test.png')

It seems related to #4243