overloading.py not compatible with python2.6

ns-3 will not find pybindgen on a Python2.6 system (CentOS 6.6) due to this incompatibility:

Checking for python module 'pybindgen'
['/usr/bin/python', '-c', "\nimport pybindgen as current_module\nversion = getattr(current_module, '__version__', None)\nif version is not None:\n\tprint(str(version))\nelse:\n\tprint('unknown version')\n"]
err: Traceback (most recent call last):
  File "<string>", line 2, in <module>
  File "/tmp/ns-3-allinone/pybindgen/pybindgen/__init__.py", line 2, in <module>
    from pybindgen.module import Module
  File "/tmp/ns-3-allinone/pybindgen/pybindgen/module.py", line 48, in <module>
    from pybindgen.function import Function, OverloadedFunction, CustomFunctionWrapper
  File "/tmp/ns-3-allinone/pybindgen/pybindgen/function.py", line 20, in <module>
    from pybindgen import overloading
  File "/tmp/ns-3-allinone/pybindgen/pybindgen/overloading.py", line 277
    docstrings_set = {wrap.docstring for wrap in self.all_wrappers if wrap.docstring is not None}
                                       ^
SyntaxError: invalid syntax

not found
from /tmp/ns-3-allinone/ns-3-dev/bindings/python: Could not find the python module 'pybindgen'

This patch fixed it for me:

--- pybindgen/overloading.py 2014-09-14 15:39:02 +0000
+++ pybindgen/overloading.py 2015-07-01 17:26:08 +0000
@@ -274,7 +274,10 @@
                             NotSupportedError):
                         pass
             # check available docstrings for the overloads
- docstrings_set = {wrap.docstring for wrap in self.all_wrappers if wrap.docstring is not None}
+ docstrings_set = {}
+ if wrap.docstring is not None:
+ for wrap in self.all_wrappers:
+ docstrings_set.add (wrap.docstring)
             docstring = None
             if len(docstrings_set) is 1:
                 docstring = docstrings_set.pop()