> I've turned ash's test program into a bunch of test cases against 
> Python 3.5 trunk.  Is it worth committing them?

Yeah, probably. Review comments...

+        try:
+            self.parser.error(RUSSIAN_TEXT)
+        except InterceptedError:
+            pass

Why not self.assertRaises()?

Also, when I run the test on its own, it prints

'''
Usage: regrtest.py [options]

regrtest.py: error: Русский текст --unknown
'''

to stderr. Probably need to fiddle with sys.stderr to fix that. Blech.

Finally:

+        try:
+            import optparse
+            old_gettext = optparse._
+            optparse._ = dummy_gettext
+
+            try:
+                OptionParser().parse_args(["--unknown"])
+            except SystemExit:
+                pass
+        finally:
+            optparse._ = old_gettext

This is a lot easier with mock.