bpo-40275: Use new test.support helper submodules in tests (GH-21743)

diff --git a/Lib/test/test_webbrowser.py b/Lib/test/test_webbrowser.py
index 6ceb490..673cc99 100644
--- a/Lib/test/test_webbrowser.py
+++ b/Lib/test/test_webbrowser.py
@@ -6,6 +6,7 @@
 from unittest import mock
 from test import support
 from test.support import import_helper
+from test.support import os_helper
 
 
 URL = 'http://www.example.com'
@@ -305,7 +306,7 @@
             browser = webbrowser.get().name
         except (webbrowser.Error, AttributeError) as err:
             self.skipTest(str(err))
-        with support.EnvironmentVarGuard() as env:
+        with os_helper.EnvironmentVarGuard() as env:
             env["BROWSER"] = browser
             webbrowser = import_helper.import_fresh_module('webbrowser')
             webbrowser.get()
@@ -318,12 +319,12 @@
         except (webbrowser.Error, AttributeError, IndexError) as err:
             self.skipTest(str(err))
 
-        with support.EnvironmentVarGuard() as env:
+        with os_helper.EnvironmentVarGuard() as env:
             env["BROWSER"] = least_preferred_browser
             webbrowser = import_helper.import_fresh_module('webbrowser')
             self.assertEqual(webbrowser.get().name, least_preferred_browser)
 
-        with support.EnvironmentVarGuard() as env:
+        with os_helper.EnvironmentVarGuard() as env:
             env["BROWSER"] = sys.executable
             webbrowser = import_helper.import_fresh_module('webbrowser')
             self.assertEqual(webbrowser.get().name, sys.executable)