Raise DeprecationWarning on raise_ioerror

[radarhere]

Helps python-pillow#4536

This is in support of 1, keep raise_ioerror and raise a deprecation warning.

I was thinking that 2, making it private, would be not ideal if someone was creating their own plugin and found some need for it.

Then I found that TiffImagePlugin raises an OSError in the same way that ImageFile does/will - https://github.com/python-pillow/Pillow/blob/a8a4b9bfdbf36ab964f57b34d1e1733d232b6175/src/PIL/TiffImagePlugin.py#L1181-L1182. If not for the fact that we are concerned with backwards compatibility, I'd suggest we change the TiffImagePlugin line from raise OSError(err) to ImageFile.raise_oserror(err).

So yes, I think the raise_oserror function could still be useful external to ImageFile.

[hugovk]

Sounds good!

Let's also add it to docs/deprecations.rst

[radarhere]

Okay, I've added another commit.

[hugovk]

Thank you!