Stop using twisted.internet.defer.returnValue (#18020)

`defer.returnValue` was only needed in Python 2; in Python 3, a simple
`return` is fine.

`twisted.internet.defer.returnValue` is deprecated as of Twisted 24.7.0.

Most uses of `returnValue` in synapse were removed a while back; this
cleans up some remaining bits.

contrib/cmdclient/console.py

@@ -245,7 +245,7 @@ class SynapseCmd(cmd.Cmd):
 
         if "flows" not in json_res:
             print("Failed to find any login flows.")
-            defer.returnValue(False)
+            return False
 
         flow = json_res["flows"][0]  # assume first is the one we want.
         if "type" not in flow or "m.login.password" != flow["type"] or "stages" in flow:
@@ -254,8 +254,8 @@ class SynapseCmd(cmd.Cmd):
                 "Unable to login via the command line client. Please visit "
                 "%s to login." % fallback_url
             )
-            defer.returnValue(False)
-        defer.returnValue(True)
+            return False
+        return True
 
     def do_emailrequest(self, line):
         """Requests the association of a third party identifier

contrib/cmdclient/http.py

@@ -78,7 +78,7 @@ class TwistedHttpClient(HttpClient):
             url, data, headers_dict={"Content-Type": ["application/json"]}
         )
         body = yield readBody(response)
-        defer.returnValue((response.code, body))
+        return response.code, body
 
     @defer.inlineCallbacks
     def get_json(self, url, args=None):
@@ -88,7 +88,7 @@ class TwistedHttpClient(HttpClient):
             url = "%s?%s" % (url, qs)
         response = yield self._create_get_request(url)
         body = yield readBody(response)
-        defer.returnValue(json.loads(body))
+        return json.loads(body)
 
     def _create_put_request(self, url, json_data, headers_dict: Optional[dict] = None):
         """Wrapper of _create_request to issue a PUT request"""
@@ -134,7 +134,7 @@ class TwistedHttpClient(HttpClient):
             response = yield self._create_request(method, url)
 
         body = yield readBody(response)
-        defer.returnValue(json.loads(body))
+        return json.loads(body)
 
     @defer.inlineCallbacks
     def _create_request(
@@ -173,7 +173,7 @@ class TwistedHttpClient(HttpClient):
         if self.verbose:
             print("Status %s %s" % (response.code, response.phrase))
             print(pformat(list(response.headers.getAllRawHeaders())))
-        defer.returnValue(response)
+        return response
 
     def sleep(self, seconds):
         d = defer.Deferred()