[cppyy] fix usage of Python GIL to support threading #22449

Job	Run time
mac14 X64 CMAKE_CXX_STANDARD=20	52m 12s
fedora42 CMAKE_CXX_STANDARD=23	29m 45s
mac-beta ARM64 CMAKE_CXX_STANDARD=23	41m 0s
mac15 ARM64 CMAKE_CXX_STANDARD=23	5m 46s
alma9 modules_off runtime_cxxmodules=Off	26m 40s
Upload Event Payload	3s
Windows 10 x64 Release	1h 7m 40s
ubuntu2404 CMAKE_BUILD_TYPE=Debug	1h 22m 49s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, builtin_zlib=ON, builtin_zstd=ON	30m 45s
Windows 10 x86 Release	1h 8m 8s
alma9 CMAKE_BUILD_TYPE=Debug	1h 7m 56s
mac13 ARM64 builtin_zlib=ON	23m 20s
ubuntu22 imt=Off, CMAKE_BUILD_TYPE=Debug	1h 7m 21s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, builtin_zlib=ON, builtin_zstd=ON	41m 15s
fedora43 CMAKE_CXX_STANDARD=23	34m 12s
alma8	31m 13s
ubuntu2504 CMAKE_CXX_STANDARD=23	35m 42s
alma10 clang Ninja CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	23m 52s
alma10	26m 38s
debian125 CMAKE_CXX_STANDARD=20, dev=ON, CMAKE_CXX_FLAGS=-Wsuggest-override	37m 34s
rawhide Fedora pydebug CMAKE_CXX_STANDARD=23	32m 36s
fedora41	37m 37s
	14h 24m 4s

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