Correct return value of round in Python 2. (#245)

In Python 2.7, round returns a float, even when ndigits is omitted. In Python 3, round returns an int if ndigits is omitted.

stdlib/2.7/__builtin__.pyi

@@ -750,7 +750,7 @@ def reversed(object: Reversible[_T]) -> Iterator[_T]: ...
 def reversed(object: Sequence[_T]) -> Iterator[_T]: ...
 def repr(o: object) -> str: ...
 @overload
-def round(number: float) -> int: ...
+def round(number: float) -> float: ...
 @overload
 def round(number: float, ndigits: int) -> float: ...  # Always return a float if given ndigits.
 @overload