Add source distribution to PyPi package.

[richardjrossiii]

Wheel releases are fine for many python deployments, but for those of us who use different python toolchains, non-binary releases would be greatly appreciated.

[AaronAtDuo]

@richardjrossiii We are unlikely to make any further updates or releases to this repository, since we encourage all our users to migrate to the new Universal client at https://github.com/duosecurity/duo_universal_python. That is also available on PyPi (https://pypi.org/project/duo-universal/) and we do have a source distribution of it available. For a guide on migrating an application, see #57.