[Python] Find the key with most items in a dictionary

	#suppose we have the following x dictionary
	>>> x
	defaultdict(None, {1: [2], 2: [1], 6: [9, 5], 7: [9], 8: [9], 9: [6, 7, 8]})
	#we want to find the key with most items
	#Python 2
	>>> sorted(x.viewitems(), key=lambda k: len(k[1]), reverse=True)
	[(9, [6, 7, 8]), (6, [9, 5]), (1, [2]), (2, [1]), (7, [9]), (8, [9])]
	>>> sorted(x.viewitems(), key=lambda k: len(k[1]), reverse=True)[0][0]
	9
	#Python3
	>>> sorted(x.items(), key=lambda k: len(k[1]), reverse=True)[0][0]
	9

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Reference: http://stackoverflow.com/questions/9363670/sort-dictionary-by-number-of-values-under-each-key