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/parser.pyi

@@ -0,0 +1,22 @@
+from _typeshed import AnyPath
+from types import CodeType
+from typing import Any, List, Sequence, Text, Tuple
+
+def expr(source: Text) -> STType: ...
+def suite(source: Text) -> STType: ...
+def sequence2st(sequence: Sequence[Any]) -> STType: ...
+def tuple2st(sequence: Sequence[Any]) -> STType: ...
+def st2list(st: STType, line_info: bool = ..., col_info: bool = ...) -> List[Any]: ...
+def st2tuple(st: STType, line_info: bool = ..., col_info: bool = ...) -> Tuple[Any]: ...
+def compilest(st: STType, filename: AnyPath = ...) -> CodeType: ...
+def isexpr(st: STType) -> bool: ...
+def issuite(st: STType) -> bool: ...
+
+class ParserError(Exception): ...
+
+class STType:
+    def compile(self, filename: AnyPath = ...) -> CodeType: ...
+    def isexpr(self) -> bool: ...
+    def issuite(self) -> bool: ...
+    def tolist(self, line_info: bool = ..., col_info: bool = ...) -> List[Any]: ...
+    def totuple(self, line_info: bool = ..., col_info: bool = ...) -> Tuple[Any]: ...