core (xmpp): Fix type hints for Tor:

The type hints were using `Tor` directly, causing a failure if `txtorcon` was not installed.
This patch fixes it by using a type hints string instead.

libervia/backend/core/xmpp.py

@@ -257,7 +257,7 @@ class SatXMPPEntity(core_types.SatXMPPEntity):
         timer["total"] += timer["connected"]
 
     @staticmethod
-    async def _prepare_tor() -> Tor:
+    async def _prepare_tor() -> "Tor":
         log.debug("We have an .onion domain, we use Tor to connect.")
         if txtorcon is None:
             raise exceptions.MissingModule(
@@ -1039,7 +1039,7 @@ class SatXMPPClient(SatXMPPEntity, core_types.SatXMPPClient, wokkel_client.XMPPC
         host=None,
         port=C.XMPP_C2S_PORT,
         max_retries=C.XMPP_MAX_RETRIES,
-        tor: Tor | None = None,
+        tor: "Tor | None" = None,
     ):
         # XXX: DNS SRV records are checked when the host is not specified.
         # If no SRV record is found, the host is directly extracted from the JID.