python2: backport fix for pyc race condition

This is python bug https://bugs.python.org/issue13146. Fixed since
python 3.4. It makes pyc creation atomic, preventing a race condition.
The patch has been rebased on our deterministic build patch.

It wasn't backported to python 2.7 because there was a complaint about
changed semantics. Since files are now created in a temporary directory
and then moved, symlinks will be overridden. See
https://bugs.python.org/issue17222.

That is an edge-case however. Ubuntu and debian have backported the fix
in 2013 already, making it mainstream enough for us to adopt.

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -79,6 +79,12 @@ let
         sha256 = "0l9rw6r5r90iybdkp3hhl2pf0h0s1izc68h5d3ywrm92pq32wz57";
       })
 
+      # Fix race-condition during pyc creation. Has a slight backwards
+      # incompatible effect: pyc symlinks will now be overridden
+      # (https://bugs.python.org/issue17222). Included in python >= 3.4,
+      # backported in debian since 2013.
+      # https://bugs.python.org/issue13146
+      ./atomic_pyc.patch
     ] ++ optionals (x11Support && stdenv.isDarwin) [
       ./use-correct-tcl-tk-on-darwin.patch
     ] ++ optionals stdenv.isLinux [