Fix token request URI

Author: Ludvik Jerabek

src/ser_mail_api/v1/client.py

@@ -41,7 +41,7 @@ def __init__(self, client_id: str, client_secret: str, region: Region = Region.U
         self._session.hooks = {"response": self.__error_handler.handler}
 
         # Deal with OAuth2
-        oauth2_client = OAuth2Client("https://mail.ser.proofpoint.com/v1/token", auth=(client_id, client_secret))
+        oauth2_client = OAuth2Client(f"https://{region.value}/v1/token", auth=(client_id, client_secret))
         oauth2_client.client_credentials({"grant_type": "client_credentials"})
         # Refresh token 5 minutes before expiration, this is documented in the API guide.
         self._session.auth = OAuth2ClientCredentialsAuth(oauth2_client,leeway=300, scope="client_credentials")