Correct typing of create_client(host, username) (#482)

The parameters `host` and `username` of `create_client` actually do
accept None values, as demonstrated by their default values being `None`
and the docstring explaining default behavior when not-set.

Correcting these types (by marking as Optional) allows users using `dsn`
or default behavior to not see type-checking errors.

Author: biggerfisch

clickhouse_connect/driver/__init__.py

@@ -13,8 +13,8 @@
 
 # pylint: disable=too-many-arguments,too-many-locals,too-many-branches
 def create_client(*,
-                  host: str = None,
-                  username: str = None,
+                  host: Optional[str] = None,
+                  username: Optional[str] = None,
                   password: str = '',
                   access_token: Optional[str] = None,
                   database: str = '__default__',