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/ensurepip/__init__.pyi

@@ -0,0 +1,25 @@
+import sys
+from typing import Optional
+
+def version() -> str: ...
+
+if sys.version_info >= (3, 0):
+    def bootstrap(
+        *,
+        root: Optional[str] = ...,
+        upgrade: bool = ...,
+        user: bool = ...,
+        altinstall: bool = ...,
+        default_pip: bool = ...,
+        verbosity: int = ...,
+    ) -> None: ...
+
+else:
+    def bootstrap(
+        root: Optional[str] = ...,
+        upgrade: bool = ...,
+        user: bool = ...,
+        altinstall: bool = ...,
+        default_pip: bool = ...,
+        verbosity: int = ...,
+    ) -> None: ...