Pushing artifacts to PyPI causes Travis CI build to fail

When pushing tags, Travis CI automatically pushes a new release of the library to PyPI.
This is working correctly, but we are trying to push artifacts for every combination in our build matrix, which causes all builds in the matrix after the first to fail, since the release is already on PyPI.

We should restrict our `deploy` step to only run once, if possible. Travis CI build stages might be helpful here: https://docs.travis-ci.com/user/build-stages

This isn't actively harmful, since pushing artifacts to PyPI is the last step in the Travis CI test script, and we will separately test the `master` branch at the same time, but it would be nice to fix this.

Inspect the repository's Travis CI configuration and the deploy step, then read the linked Travis CI build stages documentation. Verify that tagged builds publish the PyPI artifact only once and that the remaining matrix jobs no longer fail because the release already exists.

Written by the indexing model from the issue text.