Fix Python 3 round() signature (#2107)

In Python 3, the ndigits argument of round() defaults to None. If
ndigits is excluded or explicitly None, the result is always an int. If
ndigits is not None, the result should be the same type as the number.

$ cat test.py
from fractions import Fraction

print(type(round(0.1)))
print(type(round(0.1, None)))
print(type(round(0.1, 0)))

print(type(round(Fraction(1, 10))))
print(type(round(Fraction(1, 10), None)))
print(type(round(Fraction(1, 10), 0)))
$ python3 ./test.py
<class 'int'>
<class 'int'>
<class 'float'>
<class 'int'>
<class 'int'>
<class 'fractions.Fraction'>

Update the signatures to allow for an ndigits of None.

stdlib/3/builtins.pyi

@@ -143,7 +143,7 @@ class int:
     def __neg__(self) -> int: ...
     def __pos__(self) -> int: ...
     def __invert__(self) -> int: ...
-    def __round__(self, ndigits: int = ...) -> int: ...
+    def __round__(self, ndigits: Optional[int] = ...) -> int: ...
 
     def __eq__(self, x: object) -> bool: ...
     def __ne__(self, x: object) -> bool: ...
@@ -186,6 +186,8 @@ class float:
     @overload
     def __round__(self) -> int: ...
     @overload
+    def __round__(self, ndigits: None) -> int: ...
+    @overload
     def __round__(self, ndigits: int) -> float: ...
 
     def __eq__(self, x: object) -> bool: ...
@@ -912,9 +914,13 @@ def repr(o: object) -> str: ...
 @overload
 def round(number: float) -> int: ...
 @overload
-def round(number: float, ndigits: int) -> float: ...  # Always return a float if given ndigits.
+def round(number: float, ndigits: None) -> int: ...
 @overload
-def round(number: SupportsRound[_T]) -> _T: ...
+def round(number: float, ndigits: int) -> float: ...
+@overload
+def round(number: SupportsRound[_T]) -> int: ...
+@overload
+def round(number: SupportsRound[_T], ndigits: None) -> int: ...  # type: ignore
 @overload
 def round(number: SupportsRound[_T], ndigits: int) -> _T: ...
 def setattr(object: Any, name: str, value: Any) -> None: ...