7.1.1 (2020-04-02)

Fix regression seeking PNG files

This fixes a regression introduced in 7.1.0 when adding support for APNG files when calling seek and tell:

>>> from PIL import Image
>>> with Image.open("Tests/images/hopper.png") as im:
...     im.seek(0)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/PIL/PngImagePlugin.py", line 739, in seek
    if not self._seek_check(frame):
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/PIL/ImageFile.py", line 306, in _seek_check
    return self.tell() != frame
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/PIL/PngImagePlugin.py", line 827, in tell
    return self.__frame
AttributeError: 'PngImageFile' object has no attribute '_PngImageFile__frame'