pythongh-117151: increase default buffer size of shutil.copyfileobj()…

… to 256k.

it was set to 16k in the 1990s.
it was raised to 64k in 2019. the discussion at the time mentioned another 5% improvement by raising to 128k and settled for a very conservative setting.

it's 2024 now, I think it should be revisited to match modern hardware. I am measuring 0-15% performance improvement when raising to 256k on various types of disk. there is no downside as far as I can tell.

this function is only intended for sequential copy of full files (or file like objects). it's the typical use case that benefits from larger operations.

for reference, I came across this function while trying to profile pip that is using it to copy files when installing python packages.

Lib/shutil.py

@@ -44,7 +44,7 @@
 else:
     _winapi = None
 
-COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
+COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 256 * 1024
 # This should never be removed, see rationale in:
 # https://bugs.python.org/issue43743#msg393429
 _USE_CP_SENDFILE = (hasattr(os, "sendfile")