Patch #1520294: Support for getset and member descriptors in types.py,

inspect.py, and pydoc.py.  Specifically, this allows for querying the type of
an object against these built-in C types and more importantly, for getting
their docstrings printed in the interactive interpreter's help() function.

This patch includes a new built-in module called _types which provides
definitions of getset and member descriptors for use by the types.py module.
These types are exposed as types.GetSetDescriptorType and
types.MemberDescriptorType.  Query functions are provided as
inspect.isgetsetdescriptor() and inspect.ismemberdescriptor().  The
implementations of these are robust enough to work with Python implementations
other than CPython, which may not have these fundamental types.

The patch also includes documentation and test suite updates.

I commit these changes now under these guiding principles:

1. Silence is assent.  The release manager has not said "no", and of the few
   people that cared enough to respond to the thread, the worst vote was "0".

2. It's easier to ask for forgiveness than permission.

3. It's so dang easy to revert stuff in svn, that you could view this as a
   forcing function. :)

Windows build patches will follow.

Lib/types.py

@@ -86,4 +86,16 @@ EllipsisType = type(Ellipsis)
 DictProxyType = type(TypeType.__dict__)
 NotImplementedType = type(NotImplemented)
 
-del sys, _f, _g, _C, _x                  # Not for export
+# Extension types defined in a C helper module.  XXX There may be no
+# equivalent in implementations other than CPython, so it seems better to
+# leave them undefined then to set them to e.g. None.
+try:
+    import _types
+except ImportError:
+    pass
+else:
+    GetSetDescriptorType = type(_types.Helper.getter)
+    MemberDescriptorType = type(_types.Helper.member)
+    del _types
+
+del sys, _f, _g, _C, _x                           # Not for export