GeezerVison

Both _io and _pyio now use a pre-allocated buffer of length bufsize, fill it using a os.readinto / _Py_read, and have matching "expand buffer" logic.

On my machine (Linux, Debug build) this takes: ./python -m test -M8g -uall test_largefile -m test_large_read -v from ~3.7 seconds to ~3.3 seconds

_pyio still uses 2x the memory, there are two remaining copies

1. the bytes(result) currently copies. I'd like to either just rely on "duck typing" / bytearray is close enough to bytes, or would need to do something similar to C++ "move" semantics where the bytes could take ownership of the data buffer from the bytearray without copying... Not sure what is most Pythonic
2. _pyio.BufferedIO._read_unlocked in the read-all case where no buffer has been allocated always does return buf[:pos] + chunk which causes another copy. Patch for that case / "if buf is length 0, just return" coming shortly.

• Issue: Reduce copies when reading files in pyio, match behavior of _io #129005