bpo-36103: change default buffer size of shutil.copyfileobj() (GH-12115)

It is changed from 16KiB to 64KiB.  The previous default value
is used since 1990.

coreutils chose 128 KiB as minimum buffer size for block device I/O.

But shutil.copyfileobj() can be used for non block devices.
So I choose more conservative value.

As my quick benchmark, performance difference between 64KiB and
128 KiB is up to ~5%.  On the other hand, performance difference
between 32 KiB and 64 KiB can be more than 10% when file is fully
buffered.

This is why 64 KiB is rational value.

Lib/shutil.py

@@ -49,7 +49,7 @@ if os.name == 'posix':
 elif _WINDOWS:
     import nt
 
-COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 16 * 1024
+COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
 _HAS_SENDFILE = posix and hasattr(os, "sendfile")
 _HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS