Push to pypi only once per build

[johnraz]

The test matrix is trying to deploy to pypi for every version we have.

pypi legitimately errors because we try to upload multiple time the same file.

We should find a way to tell travis to only push to pypi once.

[belugame]

https://docs.travis-ci.com/user/build-stages/matrix-expansion/

this looks exactly like what we want. it tests a matrix but only deploys one item of it.

[belugame]

this also looks like it fits us with using "on:"

https://stackoverflow.com/a/41543235/640916