Cython header-only demo project should have NumPy own the data

[jpivarski]

Similar to #3261, but for the Cython example. It's using malloc to make buffers, but it should use np.empty.

awkward/header-only/examples/cython/demo_impl.cpp

Lines 40 to 44 in e946646

	// Allocate memory
	std::map<std::string, void *> buffers = {};
	for (auto it: names_nbytes) {
	buffers[it.first] = malloc(it.second);
	}

It should be even easier in Cython, since you can call Python directly.

Originally posted by @jpivarski in #3261 (comment)