builtins,numbers: harmonize float.__round__ and Real.__round__ (#3193)

See:
https://docs.python.org/3/reference/datamodel.html#object.__round__
https://docs.python.org/3/library/functions.html#round

stdlib/2/__builtin__.pyi

@@ -244,9 +244,7 @@ class float:
     def __trunc__(self) -> int: ...
     if sys.version_info >= (3,):
         @overload
-        def __round__(self) -> int: ...
-        @overload
-        def __round__(self, ndigits: None) -> int: ...
+        def __round__(self, ndigits: None = ...) -> int: ...
         @overload
         def __round__(self, ndigits: int) -> float: ...
 

stdlib/2and3/builtins.pyi

@@ -244,9 +244,7 @@ class float:
     def __trunc__(self) -> int: ...
     if sys.version_info >= (3,):
         @overload
-        def __round__(self) -> int: ...
-        @overload
-        def __round__(self, ndigits: None) -> int: ...
+        def __round__(self, ndigits: None = ...) -> int: ...
         @overload
         def __round__(self, ndigits: int) -> float: ...
 

stdlib/2and3/numbers.pyi

@@ -5,7 +5,7 @@
 # Note: these stubs are incomplete. The more complex type
 # signatures are currently omitted.
 
-from typing import Any, Optional, SupportsFloat
+from typing import Any, Optional, SupportsFloat, overload
 from abc import ABCMeta, abstractmethod
 import sys
 
@@ -70,7 +70,11 @@ class Real(Complex, SupportsFloat):
         @abstractmethod
         def __ceil__(self) -> int: ...
         @abstractmethod
-        def __round__(self, ndigits: Optional[int] = ...): ...
+        @overload
+        def __round__(self, ndigits: None = ...): ...
+        @abstractmethod
+        @overload
+        def __round__(self, ndigits: int): ...
     def __divmod__(self, other): ...
     def __rdivmod__(self, other): ...
     @abstractmethod