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/imghdr.pyi

@@ -0,0 +1,20 @@
+import os
+import sys
+from typing import Any, BinaryIO, Callable, List, Optional, Protocol, Text, Union, overload
+
+class _ReadableBinary(Protocol):
+    def tell(self) -> int: ...
+    def read(self, size: int) -> bytes: ...
+    def seek(self, offset: int) -> Any: ...
+
+if sys.version_info >= (3, 6):
+    _File = Union[Text, os.PathLike[Text], _ReadableBinary]
+else:
+    _File = Union[Text, _ReadableBinary]
+
+@overload
+def what(file: _File, h: None = ...) -> Optional[str]: ...
+@overload
+def what(file: Any, h: bytes) -> Optional[str]: ...
+
+tests: List[Callable[[bytes, Optional[BinaryIO]], Optional[str]]]