Update links in README.md

[slel]

Update the PyPI url, fixing point (a) raised in the discussion at
https://groups.google.com/d/topic/snappy-compression/iKpuTpTcjOI/discussion

Additionally, update many other urls, whether simply from http to https,
or reflecting various moves, or linking to archived versions for pages
that now give "404 Not Found" errors.

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