bpo-39999: Improve compatibility of the ast module. (GH-19056)

* Re-add removed classes Suite, slice, Param, AugLoad and AugStore.
* Add docstrings for dummy classes.
* Add docstrings for attribute aliases.
* Set __module__ to "ast" instead of "_ast".

Lib/ast.py

@@ -489,6 +489,7 @@ class NodeTransformer(NodeVisitor):
 # It will be removed in future.
 
 def _getter(self):
+    """Deprecated. Use value instead."""
     return self.value
 
 def _setter(self, value):
@@ -499,6 +500,9 @@ Constant.s = property(_getter, _setter)
 
 class _ABC(type):
 
+    def __init__(cls, *args):
+        cls.__doc__ = """Deprecated AST node class. Use ast.Constant instead"""
+
     def __instancecheck__(cls, inst):
         if not isinstance(inst, Constant):
             return False
@@ -564,15 +568,21 @@ _const_node_type_names = {
     type(...): 'Ellipsis',
 }
 
-class Index(AST):
+class slice(AST):
+    """Deprecated AST node class."""
+
+class Index(slice):
+    """Deprecated AST node class. Use the index value directly instead."""
     def __new__(cls, value, **kwargs):
         return value
 
-class ExtSlice(AST):
+class ExtSlice(slice):
+    """Deprecated AST node class. Use ast.Tuple instead."""
     def __new__(cls, dims=(), **kwargs):
         return Tuple(list(dims), Load(), **kwargs)
 
 def _dims_getter(self):
+    """Deprecated. Use elts instead."""
     return self.elts
 
 def _dims_setter(self, value):
@@ -580,6 +590,18 @@ def _dims_setter(self, value):
 
 Tuple.dims = property(_dims_getter, _dims_setter)
 
+class Suite(mod):
+    """Deprecated AST node class.  Unused in Python 3."""
+
+class AugLoad(expr_context):
+    """Deprecated AST node class.  Unused in Python 3."""
+
+class AugStore(expr_context):
+    """Deprecated AST node class.  Unused in Python 3."""
+
+class Param(expr_context):
+    """Deprecated AST node class.  Unused in Python 3."""
+
 
 # Large float and imaginary literals get turned into infinities in the AST.
 # We unparse those infinities to INFSTR.