Add type signature for a WSGI Application to wsgiref.

This type is something core to Python and is useful when typing web applications,
but doesn't actually exist in the stdlib anywhere.  I put this in wsgiref, but I am
open to suggestions as for a better place.

Author: Roy Williams

stdlib/2/wsgiref/__init__.pyi

@@ -0,0 +1,26 @@
+# Type declaration for a WSGI Function in Python 2
+#
+# This function actually exist, but we need a central place to define the type
+# of a WSGI Application.
+#
+# To correctly use this type stub, utilize the `TYPE_CHECKING` flag in
+# typing:
+#
+# from typing import TYPE_CHECKING
+#
+# if TYPE_CHECKING:
+#   from wsgiref import WSGIFunction
+#
+
+from typing import Callable, Iterable, List, Optional, Tuple, Type, Union
+from types import TracebackType
+
+exc_info = Tuple[Optional[Type[BaseException]],
+                 Optional[BaseException],
+                 Optional[TracebackType]]
+WSGIFunction = Callable[[Dict[Union[unicode, str], Union[unicode, str]],
+                         Union[
+                           Callable[[Text, List[Tuple[Union[unicode, str], Union[unicode, str]]]], Callable[[Union[unicode, str]], None]],
+                           Callable[[Text, List[Tuple[Union[unicode, str], Union[unicode, str]]], exc_info], Callable[[Union[unicode, str]], None]]
+                         ]],
+                         Iterable[Union[unicode, str]]]

stdlib/3/wsgiref/__init__.pyi

@@ -0,0 +1,26 @@
+# Type declaration for a WSGI Function in Python 3
+#
+# This function actually exist, but we need a central place to define the type
+# of a WSGI Application.
+#
+# To correctly use this type stub, utilize the `TYPE_CHECKING` flag in
+# typing:
+#
+# from typing import TYPE_CHECKING
+#
+# if TYPE_CHECKING:
+#   from wsgiref import WSGIFunction
+#
+
+from typing import Callable, Iterable, List, Optional, Tuple, Type, Union
+from types import TracebackType
+
+exc_info = Tuple[Optional[Type[BaseException]],
+                 Optional[BaseException],
+                 Optional[TracebackType]]
+WSGIFunction = Callable[[Dict[Union[bytes, str], Union[bytes, str]],
+                         Union[
+                           Callable[[Text, List[Tuple[Union[bytes, str], Union[bytes, str]]]], Callable[[Union[bytes, str]], None]],
+                           Callable[[Text, List[Tuple[Union[bytes, str], Union[bytes, str]]], exc_info], Callable[[Union[bytes, str]], None]]
+                         ]],
+                         Iterable[Union[bytes, str]]]