Log Message:
Depend on py-six.

py-OpenSSL 0.14 started depending on six, but this package didn't, so
"import OpenSSL" failed.  Confusingly, this led to build failures in
tahoe-lafs because somehow setuptools determined six was needed and
tried to download it.

After this commit, "make test" in py-OpenSSL still fails; it tries to \ 
download
"cryptography" and "cffi".