bpo-33199: Initialize ma_version_tag in PyDict_Copy (GH-6341)

Misc/NEWS.d/next/Core and Builtins/2018-04-02-09-32-40.bpo-33199.TPnxQu.rst

+Fix ``ma_version_tag`` in dict implementation is uninitialized when copying
+from key-sharing dict.

Objects/dictobject.c

@@ -2551,6 +2551,7 @@ PyDict_Copy(PyObject *o)
         split_copy->ma_values = newvalues;
         split_copy->ma_keys = mp->ma_keys;
         split_copy->ma_used = mp->ma_used;
+        split_copy->ma_version_tag = DICT_NEXT_VERSION();
         DK_INCREF(mp->ma_keys);
         for (i = 0, n = size; i < n; i++) {
             PyObject *value = mp->ma_values[i];