implement "pip self-check" with automatic invocation by default

initial work cribbed from PR #1214 but then I changed direction a little
The Accept header was carried over from Donald's implementation. I'll ping him tomorrow about it.

Warehouse will continue to support the current PyPI API endpoints for the foreseeable future.

The user-agent string idea is a good one. I'll look into appending something meaningful to the existing "python-requests/1.2.0" (or similar) header generated by requests.