bpo-40275: Use new test.support helper submodules in tests (GH-21314)

diff --git a/Lib/test/test_netrc.py b/Lib/test/test_netrc.py
index 7ce7e56..2bd46aa 100644
--- a/Lib/test/test_netrc.py
+++ b/Lib/test/test_netrc.py
@@ -1,5 +1,6 @@
 import netrc, os, unittest, sys, tempfile, textwrap
 from test import support
+from test.support import os_helper
 
 
 class NetrcTestCase(unittest.TestCase):
@@ -108,16 +109,16 @@
     def test_security(self):
         # This test is incomplete since we are normally not run as root and
         # therefore can't test the file ownership being wrong.
-        d = support.TESTFN
+        d = os_helper.TESTFN
         os.mkdir(d)
-        self.addCleanup(support.rmtree, d)
+        self.addCleanup(os_helper.rmtree, d)
         fn = os.path.join(d, '.netrc')
         with open(fn, 'wt') as f:
             f.write("""\
                 machine foo.domain.com login bar password pass
                 default login foo password pass
                 """)
-        with support.EnvironmentVarGuard() as environ:
+        with os_helper.EnvironmentVarGuard() as environ:
             environ.set('HOME', d)
             os.chmod(fn, 0o600)
             nrc = netrc.netrc()
@@ -127,10 +128,10 @@
             self.assertRaises(netrc.NetrcParseError, netrc.netrc)
 
     def test_file_not_found_in_home(self):
-        d = support.TESTFN
+        d = os_helper.TESTFN
         os.mkdir(d)
-        self.addCleanup(support.rmtree, d)
-        with support.EnvironmentVarGuard() as environ:
+        self.addCleanup(os_helper.rmtree, d)
+        with os_helper.EnvironmentVarGuard() as environ:
             environ.set('HOME', d)
             self.assertRaises(FileNotFoundError, netrc.netrc)
 
@@ -139,9 +140,9 @@
                           file='unlikely_netrc')
 
     def test_home_not_set(self):
-        fake_home = support.TESTFN
+        fake_home = os_helper.TESTFN
         os.mkdir(fake_home)
-        self.addCleanup(support.rmtree, fake_home)
+        self.addCleanup(os_helper.rmtree, fake_home)
         fake_netrc_path = os.path.join(fake_home, '.netrc')
         with open(fake_netrc_path, 'w') as f:
             f.write('machine foo.domain.com login bar password pass')
@@ -152,7 +153,7 @@
 
         def fake_expanduser(s):
             called.append(s)
-            with support.EnvironmentVarGuard() as environ:
+            with os_helper.EnvironmentVarGuard() as environ:
                 environ.set('HOME', fake_home)
                 environ.set('USERPROFILE', fake_home)
                 result = orig_expanduser(s)