compare singletons by identity not equality (closes #16712)

Patch from Serhiy Storchaka.
diff --git a/Lib/test/support.py b/Lib/test/support.py
index 014bcf5..c5640e0 100644
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -1232,7 +1232,7 @@
             try:
                 return func(*args, **kwds)
             finally:
-                if orig_tz == None:
+                if orig_tz is None:
                     del os.environ['TZ']
                 else:
                     os.environ['TZ'] = orig_tz
diff --git a/Lib/test/test_ftplib.py b/Lib/test/test_ftplib.py
index 7028f46..299a146 100644
--- a/Lib/test/test_ftplib.py
+++ b/Lib/test/test_ftplib.py
@@ -342,7 +342,7 @@
                 # http://www.mail-archive.com/openssl-users@openssl.org/msg60710.html
                 pass
             self._ssl_closing = False
-            if getattr(self, '_ccc', False) == False:
+            if getattr(self, '_ccc', False) is False:
                 super(SSLConnection, self).close()
             else:
                 pass
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 4e7a37c..f59607b 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -651,7 +651,7 @@
             groups = idg.read().strip()
             ret = idg.close()
 
-        if ret != None or not groups:
+        if ret is not None or not groups:
             raise unittest.SkipTest("need working 'id -G'")
 
         self.assertEqual(
@@ -665,7 +665,7 @@
             groups = idg.read().strip()
             ret = idg.close()
 
-        if ret != None or not groups:
+        if ret is not None or not groups:
             raise unittest.SkipTest("need working 'id -G'")
 
         # 'id -G' and 'os.getgroups()' should return the same
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 6e0c6ee..e5ec85c 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -897,7 +897,7 @@
         except TypeError:
             tb = sys.exc_info()[2]
             # traceback
-            if tb != None:
+            if tb is not None:
                 check(tb, size('2P2i'))
         # symtable entry
         # XXX