https://pymorphy2.readthedocs.io/en/stable/user/index.html

======= install ========
pip install pymorphy2
pip install -U pymorphy2-dicts-ru
pip install -U pymorphy2-dicts-uk

========= config ============
import pymorphy2
parser = pymorphy2.MorphAnalyzer()

=========== use =============
parser.parse("слон")
parser.parse("слона")[0].inflect({'sing', 'nomn'}).word