Add python-six dependency to Build-Depends in stdeb.cfg

Author: g1itch

stdeb.cfg

@@ -1,7 +1,7 @@
 [DEFAULT]
 Package: pybitmessage
 Section: net
-Build-Depends: dh-python, libssl-dev, python-all-dev, python-setuptools
+Build-Depends: dh-python, libssl-dev, python-all-dev, python-setuptools, python-six
 Depends: openssl, python-setuptools
 Recommends: apparmor, python-msgpack, python-qt4, python-stem, tor
 Suggests: python-pyopencl, python-jsonrpclib, python-defusedxml, python-qrcode