More explicit testing of pickling of user defined symbol #437

Job	Run time
build (Debug, yes, 3.12, yes, ubuntu-22.04, gcc)	5m 43s
build (Debug, yes, 3.9, yes, ubuntu-22.04, gcc)	5m 20s
build (Release, 3.13, yes, 3.11, ubuntu-22.04, gcc)	5m 1s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 24s
build (Release, 3.13, yes, gmpxx, no, ubuntu-22.04, gcc)	5m 54s
build (Debug, yes, 3.10, yes, ubuntu-22.04, gcc)	4m 1s
build (Release, yes, 3.13, ubuntu-22.04, gcc)	4m 42s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	5m 9s
build (Release, 3.9, yes, flint, yes, ubuntu-22.04, gcc)	7m 1s
build (Release, 3.9, yes, ubuntu-22.04, gcc)	5m 0s
build (Debug, yes, 3.11, yes, ubuntu-22.04, gcc)	4m 14s
build (Debug, 3.12, yes, 18, yes, yes, boostmp, -k "not integer_nthroot", ubuntu-24.04, gcc, llvm...	6m 56s
build (Release, 3.11, ubuntu-22.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	6m 28s
build (Release, 3.13, yes, ubuntu-22.04, clang)	5m 29s
build (Release, 3.13, macos-13, gcc)	7m 59s
build (Debug, 3.13, yes, 5, macos-13, clang)	7m 27s
build (Debug, 3.13, yes, yes, ubuntu-22.04, clang)	4m 1s
build (Release, 3.9, no, macos-13, clang)	6m 22s
build (Debug, 3.13, no, macos-13, gcc)	4m 34s
	1h 45m 45s

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