gh-150449: Fix versionchanged version: use 'next' not '3.16'

Per devguide markup guidelines, the version argument for versionchanged
should be 'next' during development; the release manager replaces it
with the actual version number at release time.

Author: ever0de

Doc/library/sqlite3.rst

@@ -1751,7 +1751,7 @@ Blob objects
 
    .. versionadded:: 3.11
 
-   .. versionchanged:: 3.16
+   .. versionchanged:: next
       :class:`Blob` now supports negative-step slices
       (e.g. ``blob[9:0:-2]``) for both reading and writing.