Remove vendored packages. #1126

Job	Run time
test_cygwin (39, windows-latest)	4m 52s
test_msys2_mingw (mingw64, x86_64, gcc, g++)	1m 40s
collateral (diffcov)	45s
ci_setuptools (3.10, ubuntu-latest)	3m 43s
test_msys2_mingw (mingw32, i686, gcc, g++)	1m 52s
collateral (docs)	32s
test_msys2_mingw (ucrt64, ucrt-x86_64, gcc, g++)	1m 42s
test (3.8, ubuntu-latest)	49s
test_msys2_mingw (clang64, clang-x86_64, clang, clang++)	1m 42s
test_msvc_python_mingw	1m 58s
test (3.8, macos-latest)	1m 13s
test (3.8, windows-latest)	2m 5s
test (3.12, ubuntu-latest)	1m 8s
test (3.12, macos-latest)	1m 4s
test (3.12, windows-latest)	2m 11s
test (3.9, ubuntu-latest)	50s
test (3.10, ubuntu-latest)	39s
test (3.11, ubuntu-latest)	40s
test (pypy3.10, ubuntu-latest)	1m 30s
check	2s
release	0s
	30m 57s

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