gh-85524: Raise "UnsupportedOperation" on FileIO.readall

[cmaloney]

io.UnsupportedOperation is a subclass of OSError and recommended by io.IOBase for this case; matches other read methods on io.FileIO.

While the issue was filed as a bug given how long this has raised OSError directly I'd like to treat it like a feature and have it 3.15/main only.

• Issue: FileIO.readall() should raise "UnsupportedOperation" when in "w" mode #85524

[cmaloney]

cc: @vstinner

[vstinner]

Currently, OSError(EBADF) is raised:

>>> fp=open("out", "wb", buffering=0)
>>> fp
<_io.FileIO name='out' mode='wb' closefd=True>
>>> fp.readall()
Traceback (most recent call last):
   ...
OSError: [Errno 9] Bad file descriptor

[vstinner]

Ah, _pyio already raises UnsupportedOperation:

>>> import _pyio
>>> fp=_pyio.open("out", "wb", buffering=0)
>>> fp.readall()
Traceback (most recent call last):
  ...
io.UnsupportedOperation: File not open for reading

[vstinner]

Merged, thanks. The change makes sense.

While the issue was filed as a bug given how long this has raised OSError directly I'd like to treat it like a feature and have it 3.15/main only.

I agree, there is no need to backport this change to stable branches.