pprint: fix handling of old-style classes  (#755)

We used to dispatch based on expr.__class__.__mro__, but when expr is an
old-style class we failed, because old-style classes do not have __class__
attribute.

Let's just use type(expr).__mro__ which works in all cases.

diff --git a/sympy/printing/printer.py b/sympy/printing/printer.py
--- a/sympy/printing/printer.py
+++ b/sympy/printing/printer.py
@@ -90,7 +90,7 @@ class Printer(object):
         # See if the class of expr is known, or if one of its super
         # classes is known, and use that print function
         res = None
-        for cls in expr.__class__.__mro__:
+        for cls in type(expr).__mro__:
             if hasattr(self, '_print_'+cls.__name__):
                 res = getattr(self, '_print_'+cls.__name__)(expr, *args)
                 break
diff --git a/sympy/printing/tests/test_pretty.py b/sympy/printing/tests/test_pretty.py
--- a/sympy/printing/tests/test_pretty.py
+++ b/sympy/printing/tests/test_pretty.py
@@ -215,3 +215,12 @@ def test_pretty_limits():
 def test_pretty_limits():
     assert pretty( limit(x, x, oo, evaluate=False) ) == ' lim x\nx->oo '
     assert pretty( limit(x**2, x, 0, evaluate=False) ) == '     2\nlim x \nx->0  '  
+
+def test_pretty_class():
+    """test that printer dispatcher correctly handles classes"""
+    class C: pass   # C has no .__class__ and this was causing problems
+    class D(object): pass
+
+    assert pretty( C ) == "test_pretty.C"
+    assert pretty( D ) == "<class 'test_pretty.D'>"
+