Fix error in approx's repr with complex numbers

Fix #2082

_pytest/python.py

@@ -1419,6 +1419,9 @@ class ApproxNonIterable(object):
         self.rel = rel
 
     def __repr__(self):
+        if isinstance(self.expected, complex):
+            return str(self.expected)
+
         # Infinities aren't compared using tolerances, so don't show a
         # tolerance.
         if math.isinf(self.expected):