Add a security policy. #282

Job	Run time
pre-commit	17s
list	14s
test (tests-3.8)	1m 25s
test (tests-3.9)	1m 52s
test (tests-3.10)	1m 29s
test (tests-3.11)	1m 45s
test (tests-3.12)	1m 36s
test (tests-pypy3)	1m 44s
test (readme)	1m 26s
macos (x86_64)	3m 16s
macos (aarch64)	3m 0s
musllinux (aarch64-unknown-linux-musl)	2m 27s
musllinux (i686-unknown-linux-musl)	2m 15s
musllinux (x86_64-unknown-linux-musl)	2m 37s
manylinux (x86_64)	2m 30s
manylinux (x86)	4m 43s
manylinux (aarch64)	3m 46s
manylinux (armv7)	3m 18s
manylinux (s390x)	2m 56s
manylinux (ppc64le)	3m 42s
windows (x64)	2m 46s
windows (x86)	5m 25s
Release	0s
	54m 29s

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