Add stubtest to Travis for windows (#4113)

stdlib/2and3/socket.pyi

@@ -773,6 +773,8 @@ def setdefaulttimeout(timeout: Optional[float]) -> None: ...
 if sys.version_info >= (3, 3):
     if sys.platform != "win32":
         def sethostname(name: str) -> None: ...
-    def if_nameindex() -> List[Tuple[int, str]]: ...
-    def if_nametoindex(name: str) -> int: ...
-    def if_indextoname(index: int) -> str: ...
+    # Windows added these in 3.8, but didn't have them before
+    if sys.platform != "win32" or sys.version_info >= (3, 8):
+        def if_nameindex() -> List[Tuple[int, str]]: ...
+        def if_nametoindex(name: str) -> int: ...
+        def if_indextoname(index: int) -> str: ...