Close #12326: sys.platform is now always 'linux' on Linux

On Linux, sys.platform doesn't contain the major version anymore. It is now
always 'linux', instead of 'linux2' or 'linux3' depending on the Linux version
used to build Python.
diff --git a/Doc/whatsnew/3.3.rst b/Doc/whatsnew/3.3.rst
index 79a53ff..0edf5ff 100644
--- a/Doc/whatsnew/3.3.rst
+++ b/Doc/whatsnew/3.3.rst
@@ -274,8 +274,11 @@
 This section lists previously described changes and other bugfixes
 that may require changes to your code:
 
-* Stub
-
+* Issue #12326: On Linux, sys.platform doesn't contain the major version
+  anymore. It is now always 'linux', instead of 'linux2' or 'linux3' depending
+  on the Linux version used to build Python. Replace sys.platform == 'linux2'
+  with sys.platform.startswith('linux'), or directly sys.platform == 'linux' if
+  you don't need to support older Python versions.
 
 .. Issue #11591: When :program:`python` was started with :option:`-S`,
    ``import site`` will not add site-specific paths to the module search