bpo-25094: Fix test_tools.test_sundry() on Windows (GH-8406)

When Python is installed on Windows, python -m test test_tools failed
because it tried to run Tools\scripts\2to3.py which requires an
argument. Skip this script. On other platforms or on Windows but when
run from source code (not installed), the script is called "2to3"
instead of "2to.py" and so was already skipped.

Modify also the unit test to unload all modules which have been
loaded by the test.

Lib/test/test_tools/test_sundry.py

@@ -25,15 +25,25 @@ class TestSundryScripts(unittest.TestCase):
     # scripts that use windows-only modules
     windows_only = ['win_add2path']
     # blacklisted for other reasons
-    other = ['analyze_dxp']
+    other = ['analyze_dxp', '2to3']
 
     skiplist = blacklist + whitelist + windows_only + other
 
     def test_sundry(self):
-        for fn in os.listdir(scriptsdir):
-            name = fn[:-3]
-            if fn.endswith('.py') and name not in self.skiplist:
+        old_modules = support.modules_setup()
+        try:
+            for fn in os.listdir(scriptsdir):
+                if not fn.endswith('.py'):
+                    continue
+
+                name = fn[:-3]
+                if name in self.skiplist:
+                    continue
+
                 import_tool(name)
+        finally:
+            # Unload all modules loaded in this test
+            support.modules_cleanup(*old_modules)
 
     @unittest.skipIf(sys.platform != "win32", "Windows-only test")
     def test_sundry_windows(self):