[pygobject] Skip a test with older glib

[Christoph Reiter <creiter src gnome org>]

commit 14626ee5dcf380ae94680626607742a8cc048351
Author: Christoph Reiter <creiter src gnome org>
Date:   Mon Jun 6 20:08:18 2016 +0200

    Skip a test with older glib
    
    https://bugzilla.gnome.org/show_bug.cgi?id=740301

 tests/test_docstring.py |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/tests/test_docstring.py b/tests/test_docstring.py
index aa000b4..d520b04 100644
--- a/tests/test_docstring.py
+++ b/tests/test_docstring.py
@@ -5,6 +5,7 @@ import gi.docstring
 from gi.repository import GIMarshallingTests
 from gi.repository import Gio
 from gi.repository import GObject
+from gi.repository import GLib
 
 try:
     import cairo
@@ -63,6 +64,9 @@ class Test(unittest.TestCase):
         self.assertEqual(GIMarshallingTests.boolean_return_true.__doc__,
                          'boolean_return_true() -> bool')
 
+    @unittest.skipUnless((GLib.MAJOR_VERSION, GLib.MINOR_VERSION) >= (2, 42),
+                         "nullable was added in newer glib/gi")
+    # https://bugzilla.gnome.org/show_bug.cgi?id=740301
     def test_may_return_none(self):
         self.assertEqual(Gio.File.get_basename.__doc__,
                          'get_basename(self) -> str or None')