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/copy.pyi

@@ -0,0 +1,14 @@
+from typing import Any, Dict, Optional, TypeVar
+
+_T = TypeVar("_T")
+
+# None in CPython but non-None in Jython
+PyStringMap: Any
+
+# Note: memo and _nil are internal kwargs.
+def deepcopy(x: _T, memo: Optional[Dict[int, Any]] = ..., _nil: Any = ...) -> _T: ...
+def copy(x: _T) -> _T: ...
+
+class Error(Exception): ...
+
+error = Error