Improve type hints for runtime, constant wait gens

This generators will work fine with floats for arguments. In the type
hinting world, float can be used to mean int or float.

backoff/_wait_gen.py

@@ -53,8 +53,8 @@ def fibo(max_value: Optional[int] = None) -> Generator[int, None, None]:
 
 
 def constant(
-    interval: Union[int, Iterable[int]] = 1
-) -> Generator[int, None, None]:
+    interval: Union[int, Iterable[float]] = 1
+) -> Generator[Optional[float], None, None]:
     """Generator for constant intervals.
 
     Args:
@@ -72,7 +72,10 @@ def constant(
         yield val
 
 
-def runtime(*, value: Callable[[Any], int]) -> Generator[int, None, None]:
+def runtime(
+    *,
+    value: Callable[[Any], float]
+) -> Generator[Optional[float], None, None]:
     """Generator that is based on parsing the return value or thrown
         exception of the decorated method