class_docstr: Credentials Class Unittest class.
docstr: Setup.
code: def setUp(self):
      """Setup."""
      # Credentials._singleton should be None here, but a mis-behaving
      # test may have left it intact.
      Credentials._singleton = None
  
imports: import os
  import textwrap
  import unittest
  from pathlib import Path
  from unittest.mock import MagicMock, call, mock_open, patch
  import pytest
  import requests_mock
  from testfixtures import tempdir
  from streamlit import file_util
  from streamlit.runtime.credentials import Credentials, _Activation, _verify_email, email_prompt
label_desc: Use image node_id 1 for calling the CredentialsClassTest obj's underlying member method code with example usage: obj.setUp() without return types