Remove `_vendor` directory #1122

Job	Run time
test_cygwin (39, windows-latest)	4m 21s
collateral (diffcov)	44s
ci_setuptools (3.10, ubuntu-latest)	3m 45s
collateral (docs)	42s
test (3.8, ubuntu-latest)	39s
test_msvc_python_mingw	2m 2s
test (3.8, macos-latest)	56s
test_msys2_mingw (mingw64, x86_64, gcc, g++)	1m 38s
test (3.8, windows-latest)	40s
test_msys2_mingw (mingw32, i686, gcc, g++)	1m 55s
test (3.12, ubuntu-latest)	41s
test (3.12, macos-latest)	53s
test_msys2_mingw (ucrt64, ucrt-x86_64, gcc, g++)	1m 35s
test (3.12, windows-latest)	42s
test_msys2_mingw (clang64, clang-x86_64, clang, clang++)	1m 47s
test (3.9, ubuntu-latest)	41s
test (3.10, ubuntu-latest)	51s
test (3.11, ubuntu-latest)	42s
test (pypy3.10, ubuntu-latest)	54s
check	2s
release	0s
	26m 10s

Provide feedback