gh-129005: Align FileIO.readall between _pyio and _io (#129705)

Utilize `bytearray.resize()` and `os.readinto()` to reduce copies
and match behavior of `_io.FileIO.readall()`.

There is still an extra copy which means twice the memory required
compared to FileIO because there isn't a zero-copy  path from
`bytearray` -> `bytes` currently.

On my system reading a 2 GB file:
`./python -m test -M8g -uall test_largefile -m test.test_largefile.PyLargeFileTest.test_large_read -v`

Goes from ~2.7 seconds -> ~2.2 seconds

Co-authored-by: Victor Stinner <vstinner@python.org>