gh-129005: Avoid copy in _pyio.FileIO.readinto() (#129324)

`os.read()` allocated and filled a buffer by calling `read(2)`, than that
data was copied into the user provied buffer. Read directly into the
caller's buffer instead by using `os.readinto()`.

`os.readinto()` uses `PyObject_GetBuffer()` to make sure the passed
in buffer is writeable and bytes-like, drop the manual check.