[cppyy] fix usage of Python GIL to support threading #22503

Job	Run time
Windows 10 x64 Release	1h 10m 50s
Windows 10 x86 Release	1h 3m 19s
mac15 ARM64 CMAKE_CXX_STANDARD=23	29m 19s
mac13 ARM64 builtin_zlib=ON	21m 8s
mac14 X64 CMAKE_CXX_STANDARD=20	53m 13s
Upload Event Payload	3s
mac-beta ARM64 CMAKE_CXX_STANDARD=23	40m 7s
fedora41	29m 54s
fedora42 CMAKE_CXX_STANDARD=23	31m 51s
fedora43 CMAKE_CXX_STANDARD=23	28m 29s
alma8	35m 58s
ubuntu22 imt=Off, CMAKE_BUILD_TYPE=Debug	1h 16m 26s
alma9 CMAKE_BUILD_TYPE=Debug	1h 39m 4s
ubuntu2404 CMAKE_BUILD_TYPE=Debug	1h 23m 37s
ubuntu2504 CMAKE_CXX_STANDARD=23	33m 24s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, builtin_zlib=ON, builtin_zstd=ON	32m 20s
debian125 CMAKE_CXX_STANDARD=20, dev=ON, CMAKE_CXX_FLAGS=-Wsuggest-override	31m 25s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, builtin_zlib=ON, builtin_zstd=ON	42m 6s
rawhide Fedora pydebug CMAKE_CXX_STANDARD=23	32m 3s
alma10	29m 46s
alma9 modules_off runtime_cxxmodules=Off	25m 22s
alma10 clang Ninja CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	27m 8s
	15h 16m 52s

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