For Leap, build this on modern Python as 3.6 is no longer supported.

Note that, right now, this won't fix the 15.6 build as the dependency
python-editdistance is from devel:languages:python:numeric and the fixed version
in that repo is not visible in devel:languages:python. However, this will at
least give us a working build on devel:languages:python:backports. And once the
fixed version of python-editdistance makes it into 15.6 we will also get a working
build for 15.6 over here.