sort() on sorted sets fails with numeric ordering

[tetrapus]

Tested on Python 3.6, against redis 4.0.11

>>> def test_sort(redis):
  2     redis.zadd('testing', '100', 1)
  3     redis.zadd('testing', '200', 0)
  4     return redis.sort('testing', desc=True)
>>> from redis import Redis
>>> test_sort(Redis())
[b'200', b'100']
>>> from mockredis import MockRedis
>>> test_sort(MockRedis())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in test_sort
  File "<redacted>/.venv/lib/python3.6/site-packages/mockredis/client.py", line 822, in sort
    items.sort(key=lambda x: sort_type(x[1]), reverse=bool(desc))
  File "<redacted>/.venv/lib/python3.6/site-packages/mockredis/client.py", line 822, in <lambda>
    items.sort(key=lambda x: sort_type(x[1]), reverse=bool(desc))
TypeError: float() argument must be a string or a number, not 'tuple'
float() argument must be a string or a number, not 'tuple'