Fix import used in GenericCommandsMixin.randomkey

[mudetz]

Purpose

Fix a module import used un GenericCommandsMixin.randomkey

Issue

When executing the RANDOMKEY command with FakeRedis, an exception is thrown.
AttributeError: 'builtin_function_or_method' object has no attribute 'choice'

This issue was introduced in v1.10.0

Steps to reproduce

Execute the .randomkey() method (or command) using FakeRedis

Assume this pytest test

def test_randomkey_returns_existing_key(r):
    # populate db
    r.set("foo", 1)
    r.set("bar", 2)
    r.set("baz", 3)

    # request random key
    assert r.randomkey() in ("foo", "bar", "baz")  # boom!

[cunla]

Interestingly there were no tests for this method.

Thanks for this PR - good catch