Enable nested scopes by default.

Although this is a one-character change, more work needs to be done:
the compiler can get rid of a lot of non-nested-scopes code, the
documentation needs to be updated, the future statement can be
simplified, etc.

But this change enables the nested scope semantics everywhere, and
that's the important part -- we can now test code for compatibility
with nested scopes by default.

Include/compile.h

@@ -62,7 +62,7 @@ DL_IMPORT(PyFutureFeatures *) PyNode_Future(struct _node *, char *);
 DL_IMPORT(PyCodeObject *) PyNode_CompileFlags(struct _node *, char *,
 					      PyCompilerFlags *);
 
-#define NESTED_SCOPES_DEFAULT 0
+#define NESTED_SCOPES_DEFAULT 1
 #define FUTURE_NESTED_SCOPES "nested_scopes"
 
 /* for internal use only */