Fix DeprecationWarnings found when running py.test in Python 2.7 with the -3 flag.

Running through some of my tests with the `-3` flag in python2.7 I encountered some errors within py.test itself.  This fixes those errors so we can use py.test in order to identify problems with Python 3.

_pytest/_code/source.py

@@ -52,6 +52,9 @@ class Source(object):
                 return str(self) == other
             return False
 
+    def __hash__(self):
+        return hash(self.lines)
+
     def __getitem__(self, key):
         if isinstance(key, int):
             return self.lines[key]