Add stdlib/2and3/imghdr stubs (#2318)

Ad defined in https://docs.python.org/3/library/imghdr.html.

stdlib/2and3/imghdr.pyi

@@ -0,0 +1,16 @@
+from typing import overload, Union, Text, BinaryIO, Optional, Any, List, Callable
+import sys
+import os
+
+
+if sys.version_info >= (3, 6):
+    _File = Union[Text, os.PathLike[Text], BinaryIO]
+else:
+    _File = Union[Text, BinaryIO]
+
+
+@overload
+def what(file: _File) -> Optional[str]: ...
+@overload
+def what(file: Any, h: bytes)-> Optional[str]: ...
+tests: List[Callable[[bytes, BinaryIO], Optional[str]]]