Freethreading support #340

Job	Run time
build (Debug, yes, 3.12, yes, ubuntu-20.04, gcc)	4m 6s
build (Debug, yes, 3.11, yes, ubuntu-20.04, gcc)	3m 29s
build (Debug, yes, 3.10, yes, ubuntu-20.04, gcc)	3m 26s
build (Debug, yes, 3.9, yes, ubuntu-20.04, gcc)	3m 33s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	4m 30s
build (Release, 3.8, yes, gmpxx, no, ubuntu-20.04, gcc)	4m 55s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	4m 57s
build (Release, yes, 3.8, ubuntu-20.04, gcc)	4m 23s
build (Release, 3.9, yes, ubuntu-20.04, gcc)	4m 58s
build (Release, 3.9, yes, flint, yes, ubuntu-20.04, gcc)	4m 55s
build (Debug, 3.8, yes, yes, ubuntu-20.04, clang)	3m 7s
build (Release, 3.8, yes, ubuntu-20.04, clang)	4m 4s
build (Debug, 3.10, yes, 14, yes, boostmp, -k "not integer_nthroot", ubuntu-22.04, deb http://apt...	5m 14s
build (Debug, 3.8, yes, 5, macos-13, clang)	5m 59s
build (Release, 3.9, no, macos-13, clang)	6m 44s
build (Debug, 3.8, no, macos-13, gcc)	9m 48s
build (Release, 3.8, macos-13, gcc)	6m 43s
build (Release, 3.11, ubuntu-20.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	4m 37s
	1h 29m 28s

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