bump python version on github actions to 3.12 #2887

Job	Run time
build (asio-debugging=on picker-debugging=on)	29m 3s
check headers	3m 47s
build (extensions=off logging=off streaming=off super-seeding=off share-mode=off predictive-piece...	22m 11s
clang-tidy	9m 47s
build dist (ubuntu-20.04)	16m 48s
build (crypto=gcrypt)	25m 9s
build dist (ubuntu-22.04)	14m 49s
build (mmap-disk-io=off)	28m 18s
Fuzzers	8m 13s
pre-commit	47s
Simulations	18m 10s
Tests (address-sanitizer=norecover undefined-sanitizer=norecover crypto=openssl)	16m 29s
Tests (toolset=clang logging=off address-sanitizer=norecover undefined-sanitizer=norecover)	11m 9s
Tests (thread-sanitizer=norecover crypto=openssl release debug-symbols=on cxxflags=-Wno-tsan)	18m 2s
Tests (crypto=gnutls)	11m 52s
Tests (deprecated-functions=off)	11m 29s
	4h 6m 3s

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