Ensure redis_connect_func is set on uds connection

[alisaifee]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
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• Is the new or changed code fully tested?
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Description of change

Fixes a regression introduced in #1660 when adding the redis_connect_func kwarg to the Connection base class which results in UnixDomainSocketConnection not being usable.

Testing

To reproduce on master:

>>> import redis
>>> redis.Redis.from_url("unix:///tmp/dirty.sock").ping()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    redis.Redis.from_url("unix:///tmp/dirty.sock").ping()
  File "/var/tmp/redis-py/redis/commands/core.py", line 805, in ping
    return self.execute_command("PING", **kwargs)
  File "/var/tmp/redis-py/redis/client.py", line 1156, in execute_command
    conn = self.connection or pool.get_connection(command_name, **options)
  File "/var/tmp/redis-py/redis/connection.py", line 1245, in get_connection
    connection.connect()
  File "/var/tmp/redis-py/redis/connection.py", line 606, in connect
    if self.redis_connect_func is None:
AttributeError: 'UnixDomainSocketConnection' object has no attribute 'redis_connect_func'

[chayim]

Thank you for helping bring unix domain sockets up to the same level as other connections!