Add API key instructions for PyPI uploads

[brainwane]

Travis CI's PyPI deployment docs (source: user/deployment/pypi.md) can now suggest users make and use API tokens instead of using their PyPI usernames and passwords for authentication.

Currently the API token feature is in beta; I anticipate it will be out of beta within a few months. I see a few options. Which of the following would you prefer?

1. Replace the current advice with instructions on using a PyPI API token
2. Add API token advice with a warning that it's in beta
3. Wait until the feature's out of beta, then replace the username/password instructions with API token instructions

(followup to pypi/warehouse#6211 )

[stale]

Thanks for contributing to this issue. As it has been 90 days since the last activity, we are automatically closing the issue. This is often because the request was already solved in some way and it just wasn't updated or it's no longer applicable. If that's not the case, please do feel free to either reopen this issue or open a new one. We'll gladly take a look again! You can read more here: https://blog.travis-ci.com/2018-03-09-closing-old-issues

[brainwane]

Fixed in #2514. Thanks.