Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/opcode.pyi

@@ -0,0 +1,25 @@
+import sys
+from typing import Dict, List, Optional, Sequence
+
+cmp_op: Sequence[str]
+hasconst: List[int]
+hasname: List[int]
+hasjrel: List[int]
+hasjabs: List[int]
+haslocal: List[int]
+hascompare: List[int]
+hasfree: List[int]
+opname: List[str]
+
+opmap: Dict[str, int]
+HAVE_ARGUMENT: int
+EXTENDED_ARG: int
+
+if sys.version_info >= (3, 8):
+    def stack_effect(__opcode: int, __oparg: Optional[int] = ..., *, jump: Optional[bool] = ...) -> int: ...
+
+elif sys.version_info >= (3, 4):
+    def stack_effect(__opcode: int, __oparg: Optional[int] = ...) -> int: ...
+
+if sys.version_info >= (3, 6):
+    hasnargs: List[int]