Clients: Resolve ResourceWarning rucio#5522

The `token_file` in the baseclient is not closed immediately after its usage. It
gets closed once the garbage collector deletes the FileHandler, but this happens
after an unspecified amount of time. This leads to the `ResourceWarning`.

lib/rucio/client/baseclient.py

@@ -866,8 +866,8 @@ def __read_token(self):
             return False
 
         try:
-            token_file_handler = open(self.token_file, 'r')
-            self.auth_token = token_file_handler.readline()
+            with open(self.token_file, 'r') as token_file_handler:
+                self.auth_token = token_file_handler.readline()
             self.headers['X-Rucio-Auth-Token'] = self.auth_token
         except IOError as error:
             print("I/O error({0}): {1}".format(error.errno, error.strerror))