`Path.read_bytes()` is used to read a whole file. buffering /BufferedIO is focused around making small, possibly interleaved,read/write efficient which doesn't add value in this case.On my Mac, running the benchmark:```pythonimport pyperffrom pathlib import Pathdef read_all(all_paths):    for p in all_paths:        p.read_bytes()def read_file(path_obj):    path_obj.read_bytes()all_rst = list(Path("Doc").glob("**/*.rst"))all_py = list(Path(".").glob("**/*.py"))assert all_rst, "Should have found rst files"assert all_py, "Should have found python source files"runner = pyperf.Runner()runner.bench_func("read_file_small", read_file, Path("Doc/howto/clinic.rst"))runner.bench_func("read_file_large", read_file, Path("Doc/c-api/typeobj.rst"))```before:```python.....................read_file_small: Mean +- std dev: 6.80 us +- 0.07 us.....................read_file_large: Mean +- std dev: 10.8 us +- 0.2 us````after:```python.....................read_file_small: Mean +- std dev: 5.67 us +- 0.05 us.....................read_file_large: Mean +- std dev: 9.77 us +- 0.52 us```

`Path.read_bytes()` is used to read a whole file. buffering /BufferedIO is focused around making small, possibly interleaved,read/write efficient which doesn't add value in this case.On my Mac, running the benchmark:```pythonimport pyperffrom pathlib import Pathdef read_all(all_paths):    for p in all_paths:        p.read_bytes()def read_file(path_obj):    path_obj.read_bytes()all_rst = list(Path("Doc").glob("**/*.rst"))all_py = list(Path(".").glob("**/*.py"))assert all_rst, "Should have found rst files"assert all_py, "Should have found python source files"runner = pyperf.Runner()runner.bench_func("read_file_small", read_file, Path("Doc/howto/clinic.rst"))runner.bench_func("read_file_large", read_file, Path("Doc/c-api/typeobj.rst"))```before:```python.....................read_file_small: Mean +- std dev: 6.80 us +- 0.07 us.....................read_file_large: Mean +- std dev: 10.8 us +- 0.2 us````after:```python.....................read_file_small: Mean +- std dev: 5.67 us +- 0.05 us.....................read_file_large: Mean +- std dev: 9.77 us +- 0.52 us```

NOTE: This needs a full buildbot test pass before merge, see:python#121143 (comment).1. Added `statx` to set of allowed syscall forms (Should make Raspian bot pass).2. Check that the `fd` returned from `open` is passed to all future calls. This helps ensure things like the `stat` call uses the file descriptor rather than the `filename` to avoid TOCTOU isuses.3. Update the `Path().read_bytes()` test case to additionally validate the reduction in`isatty`/`ioctl` + `seek` calls frompython#1221114. Better diagnostic assertion messagess from@gpshead, so when the test fails have first information immediately available. Makes remote CI debugging much simpler.