bpo-39320: Handle unpacking of **values in compiler (GH-18141)

* Add DICT_UPDATE and DICT_MERGE bytecodes. Use them for ** unpacking.

* Remove BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL, as they are now unused.

* Update magic number for ** unpacking opcodes.

* Update dis.rst to incorporate new bytecodes.

* Add blurb entry.

diff --git a/Include/opcode.h b/Include/opcode.h
index 1c5cd33..19944fa 100644
--- a/Include/opcode.h
+++ b/Include/opcode.h
@@ -117,8 +117,6 @@
 #define SET_ADD                 146
 #define MAP_ADD                 147
 #define LOAD_CLASSDEREF         148
-#define BUILD_MAP_UNPACK        150
-#define BUILD_MAP_UNPACK_WITH_CALL 151
 #define SETUP_ASYNC_WITH        154
 #define FORMAT_VALUE            155
 #define BUILD_CONST_KEY_MAP     156
@@ -127,6 +125,8 @@
 #define CALL_METHOD             161
 #define LIST_EXTEND             162
 #define SET_UPDATE              163
+#define DICT_MERGE              164
+#define DICT_UPDATE             165
 
 /* EXCEPT_HANDLER is a special, implicit block type which is created when
    entering an except handler. It is not an opcode but we define it here