Adding pragma warning(disable: 4522) for MSVC <= 2017. (pybind#3142)

Author: rwgk

include/pybind11/pybind11.h

@@ -19,7 +19,6 @@
 #  pragma warning(disable: 4100) // warning C4100: Unreferenced formal parameter
 #  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
 #  pragma warning(disable: 4800) // warning C4800: 'int': forcing value to bool 'true' or 'false' (performance warning)
-#  pragma warning(disable: 4522) // warning C4522: multiple assignment operators specified
 #  pragma warning(disable: 4505) // warning C4505: 'PySlice_GetIndicesEx': unreferenced local function has been removed (PyPy only)
 #elif defined(__GNUG__) && !defined(__clang__)
 #  pragma GCC diagnostic push

include/pybind11/pytypes.h

@@ -532,6 +532,10 @@ object object_or_cast(T &&o);
 // Match a PyObject*, which we want to convert directly to handle via its converting constructor
 inline handle object_or_cast(PyObject *ptr) { return ptr; }
 
+#if defined(_MSC_VER) && _MSC_VER < 1920
+#  pragma warning(push)
+#  pragma warning(disable: 4522) // warning C4522: multiple assignment operators specified
+#endif
 template <typename Policy>
 class accessor : public object_api<accessor<Policy>> {
     using key_type = typename Policy::key_type;
@@ -580,6 +584,9 @@ class accessor : public object_api<accessor<Policy>> {
     key_type key;
     mutable object cache;
 };
+#if defined(_MSC_VER) && _MSC_VER < 1920
+#  pragma warning(pop)
+#endif
 
 PYBIND11_NAMESPACE_BEGIN(accessor_policies)
 struct obj_attr {