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/codeop.pyi

@@ -0,0 +1,14 @@
+from types import CodeType
+from typing import Optional
+
+def compile_command(source: str, filename: str = ..., symbol: str = ...) -> Optional[CodeType]: ...
+
+class Compile:
+    flags: int
+    def __init__(self) -> None: ...
+    def __call__(self, source: str, filename: str, symbol: str) -> CodeType: ...
+
+class CommandCompiler:
+    compiler: Compile
+    def __init__(self) -> None: ...
+    def __call__(self, source: str, filename: str = ..., symbol: str = ...) -> Optional[CodeType]: ...