Replace std::getenv("AIDGE_CORE_WITH_PYBIND") with Py_IsInitialized()

In include/aidge/utils/Registrar.hpp, it might be possible to replace if (std::getenv("AIDGE_CORE_WITH_PYBIND")) with if (Py_IsInitialized()). Py_IsInitialized() is already used now in the DynamicAttributes class and I think it for the same reason.