add (overwrite with) mypy stubs, if available

builtins/2and3/math.pyi

@@ -34,7 +34,7 @@ def isinf(x: float) -> bool: ...
 def isnan(x: float) -> bool: ...
 def ldexp(x: float, i: int) -> float: ...
 def lgamma(x: float) -> float: ...
-def log(x: float, base: Optional[float]) -> float: ...
+def log(x: float, base: float = ...) -> float: ...
 def log10(x: float) -> float: ...
 def log1p(x: float) -> float: ...
 def modf(x: float) -> Tuple[float, float]: ...