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This is a follow up of #25063 and similar to socket.sendfile() (#17552). It provides a 20/25% speedup when copying files with shutil.copyfile(), shutil.copy() and shutil.copy2(). Differently from #25063 this is used for filesystem files only and copyfileobj() is left alone.
Unmerged #26826 is also related to this. I applied #26826 patch and built a wrapper around copy_file_range() and the speedup is basically the same. Nevertheless, even when #26826 gets merged it probably makes sense to rely on sendfile() in case copy_file_range() is not available (it was introduced in 2016) or the UNIX platform supports file-to-file copy via sendfile(2) (even though I'm not aware of any, so this would basically be Linux-only).
Some benchmarks:
 $ dd if=/dev/urandom of=/tmp/f1 bs=1K count=128
 $ time ./python -m timeit -s 'import shutil; p1 = "/tmp/f1"; p2 = "/tmp/f2"' 'shutil.copyfile(p1, p2)'
128K copy
=========
--- without patch:
 2000 loops, best of 5: 160 usec per loop
 real 0m2.353s
 user 0m0.454s
 sys 0m1.435s
--- with patch:
 2000 loops, best of 5: 187 usec per loop
 real 0m2.724s
 user 0m0.627s
 sys 0m1.634s
8MB copy
========
$ dd if=/dev/urandom of=/tmp/f1 bs=1M count=8
--- without patch:
 50 loops, best of 5: 9.51 msec per loop
 real 0m3.392s
 user 0m0.343s
 sys 0m2.478s
--- with patch:
 50 loops, best of 5: 7.75 msec per loop
 real 0m2.878s
 user 0m0.105s
 sys 0m2.187s
512MB copy
==========
--- without patch:
 1 loop, best of 5: 872 msec per loop
 real 0m5.574s
 user 0m0.402s
 sys 0m3.115s
--- with patch:
 1 loop, best of 5: 646 msec per loop
 real 0m5.475s
 user 0m0.037s
 sys 0m2.959s