Add conditional floor for Python 3.x (#380)

Author: fabianhjr

stdlib/2and3/math.pyi

@@ -29,7 +29,10 @@ def exp(x: float) -> float: ...
 def expm1(x: float) -> float: ...
 def fabs(x: float) -> float: ...
 def factorial(x: int) -> int: ...
-def floor(x: float) -> float: ...
+if sys.version_info >= (3,):
+    def floor(x: float) -> int: ...
+else:
+    def floor(x: float) -> float: ...  # type: ignore
 def fmod(x: float, y: float) -> float: ...
 def frexp(x: float) -> Tuple[float, int]: ...
 def fsum(iterable: Iterable) -> float: ...