Fix file open mode (py3)

src/mod_sso/test/httpd_integration_test.py

@@ -30,7 +30,7 @@ devnull = open(os.devnull)
 
 
 def _start_httpd(public_key, config_file):
-    with open('public.key', 'w') as fd:
+    with open('public.key', 'wb') as fd:
         fd.write(public_key)
     env = dict(os.environ)
     env['TESTROOT'] = os.getcwd()