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/pyclbr.pyi

@@ -0,0 +1,37 @@
+import sys
+from typing import Dict, List, Optional, Sequence, Union
+
+class Class:
+    module: str
+    name: str
+    super: Optional[List[Union[Class, str]]]
+    methods: Dict[str, int]
+    file: int
+    lineno: int
+
+    if sys.version_info >= (3, 7):
+        def __init__(
+            self,
+            module: str,
+            name: str,
+            super: Optional[List[Union[Class, str]]],
+            file: str,
+            lineno: int,
+            parent: Optional[Class] = ...,
+        ) -> None: ...
+    else:
+        def __init__(self, module: str, name: str, super: Optional[List[Union[Class, str]]], file: str, lineno: int) -> None: ...
+
+class Function:
+    module: str
+    name: str
+    file: int
+    lineno: int
+
+    if sys.version_info >= (3, 7):
+        def __init__(self, module: str, name: str, file: str, lineno: int, parent: Optional[Function] = ...) -> None: ...
+    else:
+        def __init__(self, module: str, name: str, file: str, lineno: int) -> None: ...
+
+def readmodule(module: str, path: Optional[Sequence[str]] = ...) -> Dict[str, Class]: ...
+def readmodule_ex(module: str, path: Optional[Sequence[str]] = ...) -> Dict[str, Union[Class, Function, List[str]]]: ...