Rework packaging, CI builds and deployments

[nocarryr]

Before enabling tests, it'd be a good idea to update the build config:

• https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/blob/e849a3f9d59bdc13572589b9f2c552b3b23234e5/.travis.yml#L35-L53
• https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/blob/e849a3f9d59bdc13572589b9f2c552b3b23234e5/.travis.yml#L10-L18

It's probably also worth reviewing the test matrix (Py versions tested against) and relax the requirement specifiers:

• https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/blob/e849a3f9d59bdc13572589b9f2c552b3b23234e5/tox.ini#L3-L5
• https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/blob/e849a3f9d59bdc13572589b9f2c552b3b23234e5/tox.ini#L33-L37

Originally posted by @nocarryr in https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/issues/1#issuecomment-459882962

[nocarryr]

https://github.com/sphinxcontrib-versioning-ng/sphinxcontrib-versioning/blob/e849a3f9d59bdc13572589b9f2c552b3b23234e5/docs/conf.py#L36-L38