bpo-41631: _ast module uses again a global state (#21961)

Partially revert commit ac46eb4ad6:
"bpo-38113: Update the Python-ast.c generator to PEP384 (gh-15957)".

Using a module state per module instance is causing subtle practical
problems.

For example, the Mercurial project replaces the __import__() function
to implement lazy import, whereas Python expected that "import _ast"
always return a fully initialized _ast module.

Add _PyAST_Fini() to clear the state at exit.

The _ast module has no state (set _astmodule.m_size to 0). Remove
astmodule_traverse(), astmodule_clear() and astmodule_free()
functions.

Lib/ast.py

@@ -497,18 +497,20 @@ class NodeTransformer(NodeVisitor):
         return node
 
 
-# The following code is for backward compatibility.
-# It will be removed in future.
+# If the ast module is loaded more than once, only add deprecated methods once
+if not hasattr(Constant, 'n'):
+    # The following code is for backward compatibility.
+    # It will be removed in future.
 
-def _getter(self):
-    """Deprecated. Use value instead."""
-    return self.value
+    def _getter(self):
+        """Deprecated. Use value instead."""
+        return self.value
 
-def _setter(self, value):
-    self.value = value
+    def _setter(self, value):
+        self.value = value
 
-Constant.n = property(_getter, _setter)
-Constant.s = property(_getter, _setter)
+    Constant.n = property(_getter, _setter)
+    Constant.s = property(_getter, _setter)
 
 class _ABC(type):
 
@@ -600,14 +602,19 @@ class ExtSlice(slice):
     def __new__(cls, dims=(), **kwargs):
         return Tuple(list(dims), Load(), **kwargs)
 
-def _dims_getter(self):
-    """Deprecated. Use elts instead."""
-    return self.elts
+# If the ast module is loaded more than once, only add deprecated methods once
+if not hasattr(Tuple, 'dims'):
+    # The following code is for backward compatibility.
+    # It will be removed in future.
 
-def _dims_setter(self, value):
-    self.elts = value
+    def _dims_getter(self):
+        """Deprecated. Use elts instead."""
+        return self.elts
 
-Tuple.dims = property(_dims_getter, _dims_setter)
+    def _dims_setter(self, value):
+        self.elts = value
+
+    Tuple.dims = property(_dims_getter, _dims_setter)
 
 class Suite(mod):
     """Deprecated AST node class.  Unused in Python 3."""