Update readme links to pypi.org

[pganssle]

Warehouse seems definitely mature enough that we should be linking to it rather than the old PyPI.

Correct me if I'm wrong @ewdurbin, but I think it's time for this.

[ewdurbin]

Risks:

• Users who's browsers do not support TLSv1.2 will not see the page, but this is inevitable as of June 30, 2018 when our CDN provider disables TLS < 1.1
• We're still changing things at a swift pace and incurring small amounts of downtime as a result.

[pganssle]

@ewdurbin Thanks for the status update. I'm not terribly worried since I doubt that the dateutil docs/github page are a major driver of traffic to the PyPI page, and people can always still google for it.