botserver testing: Move to six.assertRaisesRegex from Regexp variant.

This ensures 2/3 compatibility. The mypy test also now passes, though
the six stubs appear very minimal - maybe incorrect?

zulip_botserver/tests/test_server.py

@@ -4,6 +4,7 @@ import unittest
 from typing import Any
 from zulip_botserver import server
 from .server_test_lib import BotServerTestCase
+import six
 
 class BotServerTests(BotServerTestCase):
     class MockMessageHandler(object):
@@ -48,9 +49,10 @@ class BotServerTests(BotServerTestCase):
                 'site': 'http://localhost',
             }
         }
-        # This complains about mismatching argument types, yet they are all correct?
-        # We should investigate and file an issue in mypy.
-        self.assertRaisesRegexp(ImportError,  # type: ignore
+        # TODO: The following passes mypy, though the six stubs don't match the
+        # unittest ones, so we could file a mypy bug to improve this.
+        six.assertRaisesRegex(self,
+                              ImportError,
                               "Bot \"nonexistent-bot\" doesn't exists. Please "
                               "make sure you have set up the flaskbotrc file correctly.",
                               lambda: self.assert_bot_server_response(available_bots=available_bots,