enable configurable rate-limiting for PyPI

Rate limited requests receive a response code of 429, per RFC 6585.

Configured in pypi pillar data with:

    rate_limit:
      enable: True
      zone_size: 30m
      max_rate: 5r/s
      burst: 10
      nodelay: False

only the toplevel key, and enable are required

zone_size, max_rate, burst, and nodelay are optional