Test: Numpy Scalar Creation (#1530)

I found that the numpy array tests already contained an empty-shaped
array test, but none with data in it.

Following PEP 3118, scalars have an empty shape and ndim 0. This
works already and is now also documented/covered by a test.

tests/test_numpy_array.cpp

@@ -68,6 +68,9 @@ template <typename T, typename T2> py::handle auxiliaries(T &&r, T2 &&r2) {
     return l.release();
 }
 
+// note: declaration at local scope would create a dangling reference!
+static int data_i = 42;
+
 TEST_SUBMODULE(numpy_array, sm) {
     try { py::module::import("numpy"); }
     catch (...) { return; }
@@ -104,6 +107,8 @@ TEST_SUBMODULE(numpy_array, sm) {
 
     // test_empty_shaped_array
     sm.def("make_empty_shaped_array", [] { return py::array(py::dtype("f"), {}, {}); });
+    // test numpy scalars (empty shape, ndim==0)
+    sm.def("scalar_int", []() { return py::array(py::dtype("i"), {}, {}, &data_i); });
 
     // test_wrap
     sm.def("wrap", [](py::array a) {