Twine: Consider using urllib3 instead of requests

[sigmavirus24]

See also kennethreitz/requests#4069

[bhrutledge]

@pypa/twine-maintainers Is this something we're likely to change?

[sigmavirus24]

There's little we get from Requests as a CLI tool except overhead. We'd likely do better with urllib3 but might need different supports which I already half-wrote at one point. Regardless, Requests is barely stable these days and urllib3 has a far better track record of addressing reported security vulnerabilities.

[di]

Let's do this.