False positives from is_copy_constructible with C++17

[sgsaenger]

When PYBIND11_CPP17 is defined, detail::is_copy_constructible may give false positives for complex structures, leading to compiler errors.

py::cast(std::map<int, std::unique_ptr<int>>{}); // does compile
py::cast(std::map<int, std::map<int, std::unique_ptr<int>>>{}); // does not compile

Undefining the macro, respectively unguarding the third variant of is_copy_constructible solves this problem.
Tested with gcc and clang(libc++) and pybind v2.3.0.

[wangxf123456]

This might still be a problem for other complex structures right now:

struct vector_unique_ptr_owner {
  std::vector<std::unique_ptr<int>> vec;
};

void try_static_assert() {
  static_assert(
    !pybind11::detail::is_copy_constructible<vector_unique_ptr_owner>::value,
    "Needs to be false");  // Fails

One workaround for this is to explicitly tell pybind11 that the object is not copy constructible:

struct vector_unique_ptr_owner {
  std::vector<std::unique_ptr<int>> vec;
};

namespace pybind11 {
namespace detail {

template <>
struct is_copy_constructible<vector_unique_ptr_owner>: std::false_type {};

}  // namespace detail
}  // namespace pybind11

void try_static_assert() {
  static_assert(
      !py::detail::is_copy_constructible<vector_unique_ptr_owner>::value,
      "Needs to be false");  // Compiles
}