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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 slackpkg_update(self):
 """This replace slackpkg ChangeLog.txt file with new from Slackware official mirrors after update distribution. """

        NEW_ChangeLog_txt = URL(mirrors("ChangeLog.txt", "")).reading()
        if os.path.isfile(self.meta.slackpkg_lib_path + "ChangeLog.txt.old"):
            os.remove(self.meta.slackpkg_lib_path + "ChangeLog.txt.old")
        if os.path.isfile(self.meta.slackpkg_lib_path + "ChangeLog.txt"):
            shutil.copy2(self.meta.slackpkg_lib_path + "ChangeLog.txt",
                         self.meta.slackpkg_lib_path + "ChangeLog.txt.old")
            os.remove(self.meta.slackpkg_lib_path + "ChangeLog.txt")
        with open(self.meta.slackpkg_lib_path + "ChangeLog.txt", "w") as log:
            log.write(NEW_ChangeLog_txt)
            log.close()