package/python-treq: drop python2 support
Fixes:
http://autobuild.buildroot.net/results/94e/94e009e322787bbcaac3704b64218cefe77b0f56/
Treq is built on top of twisted, which is python3 only since commit
f1db1922e460d4f (package/python-twisted: Needs python 3), so propagate the
python3 dependency.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>