add timespec arg to datetime.time.isoformat (#3432)

Close #3431

stdlib/2and3/datetime.pyi

@@ -114,7 +114,10 @@ class time:
     def __ge__(self, other: time) -> bool: ...
     def __gt__(self, other: time) -> bool: ...
     def __hash__(self) -> int: ...
-    def isoformat(self) -> str: ...
+    if sys.version_info >= (3, 6):
+        def isoformat(self, timespec: str = ...) -> str: ...
+    else:
+        def isoformat(self) -> str: ...
     if sys.version_info >= (3, 7):
         @classmethod
         def fromisoformat(cls: Type[_S], time_string: str) -> _S: ...