docstr: Test that setting the progress handler to None clears the previously set handler.
code: def test_clear_handler(self):
      """
      Test that setting the progress handler to None clears the previously set handler.
      """
      con = self.con
      action = 0
  
      def progress():
          nonlocal action
          action = 1
          return 0
  
      con.set_progress_handler(progress, 1)
      con.set_progress_handler(None, 1)
      con.execute("select 1 union select 2 union select 3").fetchall()
      self.assertEqual(action, 0, "progress handler was not cleared")
  
imports: import contextlib
  import sqlite3
  import unittest
  from test.support.os_helper import TESTFN, unlink
  from .util import memory_database, cx_limit, with_tracebacks
  from .util import MemoryDatabaseMixin
label_desc: Use image node_id 4 for calling the ProgressTests obj's underlying member method code with example usage: obj.test_clear_handler() and returns: int