pymorphy

First things first:

sudo pip install pymorphy

Now we need dictionaries https://bitbucket.org/kmike/pymorphy/downloads/

wget https://bitbucket.org/kmike/pymorphy/downloads/ru.sqlite-json.zip
unzip ru.sqlite-json.zip

And here is sample script:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

def normalize(word):
	from pymorphy import get_morph

	morph_ru = get_morph('dicts/ru') # path to extracted dictionary

	#word = u'менеджером'
	res = morph_ru.normalize(word.upper())

	for i in res:
		return i.lower()

if __name__ == "__main__":
	import argparse
	parser = argparse.ArgumentParser(description = 'Normalize word')
	parser.add_argument('word', help = 'word to normalize')
	args = parser.parse_args()
	print normalize(args.word.decode('utf-8'))

Example:

$ ./normalize.py менеджерам
менеджер
$ ./normalize.py менеджеру
менеджер