Enable automatic PyPI deployment on tag cut

Hi,

I just wanted to point you out that there's an option of having new package version automatically deployed to [PyPI deployment](https://docs.travis-ci.com/user/deployment/pypi/) once you release a new tag.

Corresponding section in `.travis.yml` would look as follows:
```yaml
deploy:
on:
tags: true
provider: pypi
distributions: sdist bdist_wheel
user: dblack
password:
secure: [Your encrypted password](https://docs.travis-ci.com/user/encryption-keys/)
```

*Don't forget to [encrypt the password](https://docs.travis-ci.com/user/encryption-keys/).