You should use the secrets module to generate cryptographically safe passwords, which is available starting in Python 3.6. Adapted from the documentation:
import secrets
import string
alphabet = string.ascii_letters + string.digits
password = ''.join(secrets.choice(alphabet) for i in range(20)) # for a 20-character password
For more information on recipes and best practices, see this section on recipes in the Python documentation. You can also consider adding string.punctuation
or even just using string.printable
for a wider set of characters.