Add a python_requires to setup.py

This means pip install diffoscope will not try to download new source distributions that then fail to build, if you run pip with an unsupported Python version.

(Unfortunately pip will still attempt to download older releases of diffoscope that did not have python_requires in the metadata, and those will then fail. Still, it'll be useful when you eventually drop support for Python 3.5 and start requiring newer Pythons.)