MaildirMessage.set_date accepts a float argument (#3617)

stdlib/2and3/mailbox.pyi

@@ -130,7 +130,7 @@ class MaildirMessage(Message):
     def add_flag(self, flag: str) -> None: ...
     def remove_flag(self, flag: str) -> None: ...
     def get_date(self) -> int: ...
-    def set_date(self, date: int) -> None: ...
+    def set_date(self, date: float) -> None: ...
     def get_info(self) -> str: ...
     def set_info(self, info: str) -> None: ...