Deprecated old modules, and slated them for removal in 4.0
diff --git a/doc/upgrading.rst b/doc/upgrading.rst
index c8a2d82..c21fa4b 100644
--- a/doc/upgrading.rst
+++ b/doc/upgrading.rst
@@ -3,7 +3,7 @@
Previous versions of Python-RSA were less secure than the current
version. In order to be able to gradually upgrade your software, those
-old versions are still available.
+old versions will be available until Python-RSA 4.0.
To use version 1.3.3, use this::
@@ -26,6 +26,12 @@
Those import statements *will create warnings* as they import much
less secure code into your project.
+.. warning::
+
+ These modules are included to allow upgrading to the latest version
+ of Python-RSA, and not as a way to keep using those old versions.
+ They will be removed in version 4.0.
+
The random padding introduced in version 3.0 made things much more
secure, but also requires a larger key to encrypt the same message.
You can either generate a new key with :py:func:`rsa.newkeys`, or use