[server] fix generate_source_config regression on python2
lgc outputs "str" objects on python2 and python3, so we can't just use
io.StringIO (which expects unicode objects).
Fixes: c36cda9074c5
--- a/server/serverconfig.py Wed Oct 14 15:49:03 2015 +0200
+++ b/server/serverconfig.py Mon Oct 26 15:22:09 2015 +0100
@@ -22,7 +22,8 @@
import sys
from os.path import join, exists
-from io import StringIO
+
+from six.moves import StringIO
import logilab.common.configuration as lgconfig
from logilab.common.decorators import cached