I don't know when this changed, but I think this bug is fixed now, if I rebuild 3.8.0-1 in sid, I get extension modules for 3.5 and 3.6 built:
$ sbuild -d sid lxml_3.8.0-1 ... python3-lxml_3.8.0-1_amd64.deb ------------------------------ ... -rw-r--r-- root/root 2165928 2017-06-21 01:25 ./usr/lib/python3/dist-packages/lxml/etree.cpython-35m-x86_64-linux-gnu.so -rw-r--r-- root/root 2190504 2017-06-21 01:25 ./usr/lib/python3/dist-packages/lxml/etree.cpython-36m-x86_64-linux-gnu.so ... -rw-r--r-- root/root 429896 2017-06-21 01:25 ./usr/lib/python3/dist-packages/lxml/ objectify.cpython-35m-x86_64-linux-gnu.so -rw-r--r-- root/root 438088 2017-06-21 01:25 ./usr/lib/python3/dist-packages/lxml/ objectify.cpython-36m-x86_64-linux-gnu.so ...
