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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def last_modified(self) -> str: 		""" 		Return an ISO8601 timestamp indicating when the file was last modified 		Returns: 			An ISO8601 timestamp indicating when the file was last modified 		Remarks: 			If the file has no timestamp, and mode is 'r+', a new timestamp is created and returned. 			Otherwise, the current time in UTC is returned 		"""

		if "last_modified" in self.attrs:
			return self.attrs["last_modified"]
		elif self.mode == "r+":
			# Make sure the file has modification timestamps
			self.attrs["last_modified"] = timestamp()
			return self.attrs["last_modified"]
		return timestamp()