Extracted _copy_one function. #1147

Job	Run time
test_cygwin (39, windows-latest)	4m 47s
ci_setuptools (3.10, ubuntu-latest)	3m 52s
collateral (diffcov)	46s
test (3.8, ubuntu-latest)	40s
test_msvc_python_mingw	2m 20s
collateral (docs)	33s
test (3.8, macos-latest)	1m 11s
test_msys2_mingw (mingw64, x86_64, gcc, g++)	1m 34s
test (3.8, windows-latest)	2m 14s
test_msys2_mingw (mingw32, i686, gcc, g++)	2m 0s
test (3.12, ubuntu-latest)	44s
test_msys2_mingw (ucrt64, ucrt-x86_64, gcc, g++)	1m 38s
test (3.12, macos-latest)	50s
test_msys2_mingw (clang64, clang-x86_64, clang, clang++)	1m 43s
test (3.12, windows-latest)	2m 22s
test (3.9, ubuntu-latest)	45s
test (3.10, ubuntu-latest)	37s
test (3.11, ubuntu-latest)	38s
test (pypy3.10, ubuntu-latest)	1m 38s
check	2s
release	0s
	30m 54s

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