GH-120754: Disable buffering in Path.read_bytes (#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
```

1 files changed, 1 insertions, 1 deletions

diff --git a/Lib/pathlib/_abc.py b/Lib/pathlib/_abc.py
index 500846d..720756c 100644
--- a/Lib/pathlib/_abc.py
+++ b/Lib/pathlib/_abc.py
@@ -585,7 +585,7 @@ class PathBase(PurePathBase):
         """
         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):