pythonGH-120754: Disable buffering in Path.read_bytes (python#122111)

`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:

```python
import pyperf
from pathlib import Path

def 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
```

Lib/pathlib/_abc.py

@@ -585,7 +585,7 @@ def read_bytes(self):
         """
         Open the file in bytes mode, read it, and close the file.
         """
-        with self.open(mode='rb') as f:
+        with self.open(mode='rb', buffering=0) as f:
             return f.read()
 
     def read_text(self, encoding=None, errors=None, newline=None):

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -1464,7 +1464,7 @@ def stat(self, *, follow_symlinks=True):
 
     def open(self, mode='r', buffering=-1, encoding=None,
              errors=None, newline=None):
-        if buffering != -1:
+        if buffering != -1 and not (buffering == 0 and 'b' in mode):
             raise NotImplementedError
         path_obj = self.resolve()
         path = str(path_obj)

Misc/NEWS.d/next/Library/2024-07-22-08-57-28.gh-issue-120754.Eo5puP.rst

@@ -0,0 +1 @@
+``Pathlib.read_bytes`` no longer opens the file in Python's buffered I/O mode. This reduces overheads as the code reads a file in whole leading to a modest speedup.