introduce mutex to avoid multiple token refresh when using threads.

mdrsclient/connection.py

@@ -1,3 +1,5 @@
+import threading
+
 import requests
 
 from mdrsclient.cache import CacheFile
@@ -8,12 +10,14 @@ from mdrsclient.models import Laboratories, Token, User
 class MDRSConnection:
     url: str
     session: requests.Session
+    lock: threading.Lock
     __cache: CacheFile
 
     def __init__(self, remote: str, url: str) -> None:
         super().__init__()
         self.url = url
         self.session = requests.Session()
+        self.lock = threading.Lock()
         self.__cache = CacheFile(remote)
         self.__prepare_headers()