bump to v2.3.1 throughout, as per devnotes #458

Job	Run time
prepare	2s
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 26s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	1m 6s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	1m 0s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 37s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 8s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	1m 3s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	58s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	13m 22s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	12m 42s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 36s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	13m 10s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	11m 45s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	12m 38s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 27s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	11m 48s
Build with Pip (macos-13, 3.8, llvm, -march=native)	4m 29s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	2m 58s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	1m 59s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	1m 53s
Build with Pip (macos-13, 3.11, llvm, -march=native)	2m 53s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	3m 37s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	1m 46s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	2m 44s
	2h 12m 27s

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