Fix broken CI

clang 3.x requires declaration of default copy constructors,
due to use of std::is_copy_constructible.

Author: rhaschke

tests/test_class.cpp

@@ -421,6 +421,7 @@ TEST_SUBMODULE(class_, m) {
     // test_exception_rvalue_abort
     struct PyPrintDestructor {
         PyPrintDestructor() = default;
+        PyPrintDestructor(const PyPrintDestructor&) = default;
         ~PyPrintDestructor() {
             py::print("Print from destructor");
         }

tests/test_numpy_array.cpp

@@ -206,6 +206,7 @@ TEST_SUBMODULE(numpy_array, sm) {
     struct ArrayClass {
         int data[2] = { 1, 2 };
         ArrayClass() { py::print("ArrayClass()"); }
+        ArrayClass(const ArrayClass&) = default;
         ~ArrayClass() { py::print("~ArrayClass()"); }
     };
     py::class_<ArrayClass>(sm, "ArrayClass")

tests/test_smart_ptr.cpp

@@ -161,6 +161,7 @@ class MyObject4b : public MyObject4a {
 class MyObject5 { // managed by huge_unique_ptr
 public:
     MyObject5(int value) : value{value} { print_created(this); }
+    MyObject5(const MyObject5&) = default;
     ~MyObject5() { print_destroyed(this); }
     int value;
 };
@@ -220,6 +221,7 @@ struct TypeForHolderWithAddressOf {
 // test_move_only_holder_with_addressof_operator
 struct TypeForMoveOnlyHolderWithAddressOf {
     TypeForMoveOnlyHolderWithAddressOf(int value) : value{value} { print_created(this); }
+    TypeForMoveOnlyHolderWithAddressOf(const TypeForMoveOnlyHolderWithAddressOf&) = default;
     ~TypeForMoveOnlyHolderWithAddressOf() { print_destroyed(this); }
     std::string toString() const {
         return "MoveOnlyHolderWithAddressOf[" + std::to_string(value) + "]";