WASHINGTON, April 11 (Reuters) - Longtime Republican congressman Thomas Petri of Wisconsin will not seek re-election in November, announcing his decision just a week after a conservative state legislator revealed plans to mount a primary challenge for his seat.Petri, 73, viewed as a moderate member of the party, will formally announce his retirement plans at a town hall meeting on Monday in Neenah, Wisconsin, his office said in a statement on Friday. It gave no details about the reason for the decision.Glenn Grothman, a Wisconsin state senator who is considered one of the state's most conservative Republicans, said last week...