make dnspython installable with pip

make dnspython installable with pip

I tried installing dnspython from the master branch and it did not work.
The error I got was
"poetry.masonry.utils.module.ModuleOrPackageNotFound: No file/folder
found for package dnspython".
Turns out there have been some changes made to pip involving the build
system (see https://www.python.org/dev/peps/pep-0517).
Basically pip is using poetry to install your package.
Poetry has some requirements about how you structure your project (see
https://python-poetry.org/docs/basic-usage/#project-setup and
https://python-poetry.org/docs/pyproject/#packages).
This change allows you to install dnspython without running into that
particular issue.