pythonPackages.mezzanine: Fix template project

This makes sure that the template project can be used and updated.
Otherzise files copied from the store stay readonly and cannot be
updated during the generation process.

pkgs/development/python-modules/mezzanine/writable_settings.patch

@@ -0,0 +1,21 @@
+diff -Nur mezzanine-3.1.10/mezzanine/bin/mezzanine_project.py mezzanine-3.1.10-patched/mezzanine/bin/mezzanine_project.py
+--- mezzanine-3.1.10/mezzanine/bin/mezzanine_project.py	2014-08-30 07:12:19.000000000 +0200
++++ mezzanine-3.1.10-patched/mezzanine/bin/mezzanine_project.py	2016-10-31 14:47:30.982401818 +0100
+@@ -5,6 +5,7 @@
+ from distutils.dir_util import copy_tree
+ from optparse import OptionParser
+ import os
++import stat
+ from shutil import move
+ from uuid import uuid4
+ 
+@@ -61,6 +62,9 @@
+         copy_tree(os.path.join(package_path, "project_template"), project_path)
+         move(local_settings_path + ".template", local_settings_path)
+ 
++    os.chmod(local_settings_path,
++             os.stat(local_settings_path).st_mode | stat.S_IWRITE)
++
+     # Generate a unique SECRET_KEY for the project's setttings module.
+     with open(local_settings_path, "r") as f:
+         data = f.read()

pkgs/top-level/python-packages.nix

@@ -13967,6 +13967,7 @@ in {
       url = "https://github.com/stephenmcd/mezzanine/archive/${version}.tar.gz";
       sha256 = "1cd7d3dji8q4mvcnf9asxn8j109pd5g5d5shr6xvn0iwr35qprgi";
     };
+    patches = [ ../development/python-modules/mezzanine/writable_settings.patch ];
 
     disabled = isPyPy;