Explicitly set to None to have consistent behavior in Python 2 and Python 3

_pytest/_code/code.py

@@ -29,8 +29,7 @@ class Code(object):
     def __eq__(self, other):
         return self.raw == other.raw
 
-    def __hash__(self):
-        return hash(self.raw)
+    __hash__ = None
 
     def __ne__(self, other):
         return not self == other

_pytest/_code/source.py

@@ -52,8 +52,7 @@ class Source(object):
                 return str(self) == other
             return False
 
-    def __hash__(self):
-        return hash(self.lines)
+    __hash__ = None
 
     def __getitem__(self, key):
         if isinstance(key, int):

_pytest/python.py

@@ -1356,8 +1356,7 @@ class approx(object):
             return False
         return all(a == x for a, x in zip(actual, self.expected))
 
-    def __hash__(self):
-        return hash(self.expected)
+    __hash__ = None
 
     def __ne__(self, actual):
         return not (actual == self)
@@ -1438,8 +1437,7 @@ class ApproxNonIterable(object):
         # Return true if the two numbers are within the tolerance.
         return abs(self.expected - actual) <= self.tolerance
 
-    def __hash__(self):
-        return hash((self.expected, self.tolerance))
+    __hash__ = None
 
     def __ne__(self, actual):
         return not (actual == self)