Require implementations for warnings.showwarning() support the 'line' argument.
Was a DeprecationWarning for not supporting it since Python 2.6.

Closes issue #3652.
diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index 81e7452..091327f 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -291,9 +291,8 @@
    message; if *line* is not supplied, :func:`showwarning` will
    try to read the line specified by *filename* and *lineno*.
 
-   .. versionchanged:: 2.6
-      Added the *line* argument. Implementations that lack the new argument
-      will trigger a :exc:`DeprecationWarning`.
+   .. versionchanged:: 2.7
+      The *line* argument is required to be supported.
 
 
 .. function:: formatwarning(message, category, filename, lineno[, line])