Add missing 'six' dependency

[jstastny]

six is directly imported in https://github.com/cloudflare/sqlalchemy-clickhouse/blob/master/connector.py#L106.

Without this, the example.py fails to run with:

$ python3 example.py
Traceback (most recent call last):
  File "/Users/honza/projects/sqlalchemy-clickhouse/example.py", line 4, in <module>
    import connector
  File "/Users/honza/projects/sqlalchemy-clickhouse/connector.py", line 105, in <module>
    from six import PY3, string_types
ModuleNotFoundError: No module named 'six'