Starting from setuptools-dso 1.7 pyx.build no longer misses to
(re)compile dsos when `build_ext -i` is run:

https://github.com/mdavidsaver/setuptools_dso/pull/10
https://github.com/mdavidsaver/setuptools_dso/commit/84e06348

Previously `pip install -e` would just fail when run afresh because DSOs
were not built, and so link ext->dso was failing.