Fixed collection of classes with custom `__new__` method

_pytest/python.py

@@ -662,6 +662,10 @@ class Class(PyCollector):
             self.warn("C1", "cannot collect test class %r because it has a "
                 "__init__ constructor" % self.obj.__name__)
             return []
+        elif hasnew(self.obj):
+            self.warn("C1", "cannot collect test class %r because it has a "
+                            "__new__ constructor" % self.obj.__name__)
+            return []
         return [self._getcustomclass("Instance")(name="()", parent=self)]
 
     def setup(self):
@@ -679,8 +683,7 @@ class Class(PyCollector):
 
 class Instance(PyCollector):
     def _getobj(self):
-        obj = self.parent.obj()
-        return obj
+        return self.parent.obj()
 
     def collect(self):
         self.session._fixturemanager.parsefactories(self)
@@ -793,10 +796,14 @@ class Generator(FunctionMixin, PyCollector):
 def hasinit(obj):
     init = getattr(obj, '__init__', None)
     if init:
-        if init != object.__init__:
-            return True
+        return init != object.__init__
 
 
+def hasnew(obj):
+    new = getattr(obj, '__new__', None)
+    if new:
+        return new != object.__new__
+
 
 def fillfixtures(function):
     """ fill missing funcargs for a test function. """