Properly import ThingServerInterface in thing

ThingServerInterface is used as a type hint for `Thing._thing_server_interface`. It was previously imported with an `if TYPE_CHECKING` guard, but as I've now explicitly type hinted and documented that attribute, any time `get_type_hints` is called on a `Thing` subclass (which
is every time one is defined) it will cause an error.

I've now imported it unconditionally, which fixes the problem. This does not cause any interdependency issues, as `thing_server_interface` has few onward dependencies.

Author: rwb27

src/labthings_fastapi/thing.py

@@ -29,12 +29,12 @@
 from .utilities.introspection import get_summary, get_docstring
 from .websockets import websocket_endpoint
 from .exceptions import PropertyNotObservableError
+from .thing_server_interface import ThingServerInterface
 
 
 if TYPE_CHECKING:
     from .server import ThingServer
     from .actions import ActionManager
-    from .thing_server_interface import ThingServerInterface
 
 _LOGGER = logging.getLogger(__name__)