Add type annotation to client_for_basic_auth

The lack of annotation was masking the `MarklogicApiClient` instance
from type checking. By adding this, mypy will correctly warn us if we're
committing type violations.

src/openapi_server/connect.py

@@ -13,7 +13,7 @@
 
 def client_for_basic_auth(
     token_basic: TokenModel = Security(get_token_basic),
-):
+) -> MarklogicApiClient:
     return MarklogicApiClient(
         host=MARKLOGIC_HOST,
         username=token_basic.username,