old_contents: from unittest import mock, TestCase
  
  from zabbixdocker.main import Application
  
  
  class TestApplication(TestCase):
      def test_singleton(self):
          app1 = Application()
          app2 = Application()
  
          assert app1 == app2
  
new_contents: from unittest import TestCase
  
  from zabbixdocker.main import Application
  
  
  class TestApplication(TestCase):
      def test_singleton(self):
          app1 = Application()
          app2 = Application()
  
          assert app1 == app2
  
subject: Fix flake8