Update PyPI URLs and simplify contributor instructions

[brainwane]

Migrating to the new PyPI means trying to point to pypi.org URLs whenever reasonable.

I also simplified some contributor instructions along the way.

[codecov]

Codecov Report

Merging #318 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   67.64%   67.64%           
=======================================
  Files          12       12           
  Lines         578      578           
  Branches       91       91           
=======================================
  Hits          391      391           
  Misses        162      162           
  Partials       25       25

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