Don't use 'with assert_raises', it doesn't work on Python 2.6

Author: mdboom

lib/matplotlib/tests/test_path.py

@@ -4,5 +4,7 @@
 def test_readonly_path():
     path = Path.unit_circle()
 
-    with assert_raises(AttributeError):
+    def modify_vertices():
         path.vertices = path.vertices * 2.0
+
+    assert_raises(AttributeError, modify_vertices)