input: Write a python function to delete a node from a singly linked list.
  assert llist.DeleteNode('Mon') is None
  assert llist.DeleteNode('Tue') is None
  assert llist.DeleteNode('Wed') is None
output: class Node:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  class LinkedList:
      def __init__(self):
          self.head = None
  
      def append(self, data):
          new_node = Node(data)
          if not self.head:
              self.head = new_node
              return
          last = self.head
          while last.next:
              last = last.next
          last.next = new_node
  
      def DeleteNode(self, key):
          temp = self.head
  
          if temp is not None:
              if temp.data == key:
                  self.head = temp.next
                  temp = None
                  return
  
          while temp is not None:
              if temp.data == key:
                  break
              prev = temp
              temp = temp.next
  
          if temp == None:
              return
  
          prev.next = temp.next
          temp = None