Use newer setuptools version for py36
Use newer setuptools version for py36

Unfortunately, Travis CI uses a pretty outdated version of setuptools
package in Python 3.6 environments. That version has a bug that prevents
using namespace packages [1]. Fortunately, it has been fixed in newer
versions, thus we simply must update a setuptools version.

https://github.com/pypa/setuptools/pull/1402/

ikalnytskyi/sphinxcontrib-openapi