bump to v2.3.1 throughout, as per devnotes #457

Job	Run time
prepare	3s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 20s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 12s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	1m 10s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	1m 2s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 17s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 10s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	1m 1s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	1m 6s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 54s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	11m 22s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 40s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 6s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	11m 54s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	12m 18s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 38s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	14m 56s
Build with Pip (macos-13, 3.8, llvm, -march=native)	3m 54s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	4m 39s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	2m 0s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	2m 25s
Build with Pip (macos-13, 3.11, llvm, -march=native)	3m 5s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	2m 52s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	2m 8s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	1m 42s
	2h 11m 54s

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