Update a bunch of stubs

builtins/2and3/math.pyi

@@ -3,6 +3,8 @@
 
 from typing import Tuple, Iterable, Optional
 
+import sys
+
 e = ...  # type: float
 pi = ...  # type: float
 
@@ -31,7 +33,8 @@ def fsum(iterable: Iterable) -> float: ...
 def gamma(x: float) -> float: ...
 def hypot(x: float, y: float) -> float: ...
 def isinf(x: float) -> bool: ...
-def isfinite(x: float) -> bool: ...
+if sys.version_info[0] >= 3:
+    def isfinite(x: float) -> bool: ...
 def isnan(x: float) -> bool: ...
 def ldexp(x: float, i: int) -> float: ...
 def lgamma(x: float) -> float: ...