Allow arbitrary overriding of __new__

This partially addresses #794.

Author: JukkaL

mypy/checker.py

@@ -761,8 +761,8 @@ def check_method_or_accessor_override_for_base(self, defn: FuncBase,
         """Check if method definition is compatible with a base class."""
         if base:
             name = defn.name()
-            if name != '__init__':
-                # Check method override (__init__ is special).
+            if name not in ('__init__', '__new__'):
+                # Check method override (__init__ and __new__ are special).
                 self.check_method_override_for_base_with_name(defn, name, base)
                 if name in nodes.inplace_operator_methods:
                     # Figure out the name of the corresponding operator method.

mypy/test/data/check-classes.test

@@ -296,6 +296,15 @@ main: note: In class "B":
 main:6: error: Return type of "f" incompatible with supertype "A"
 main:7: error: Return type of "g" incompatible with supertype "A"
 
+[case testOverride__new__WithDifferentSignature]
+class A:
+    def __new__(cls, x: int) -> str:
+        return ''
+
+class B(A):
+    def __new__(cls) -> int:
+        return 1
+
 
 -- Constructors
 -- ------------