Fix description in setup script

[tlestang]

For mysterious reasons, the readme disappeared from the setup.py.
This adds it back as the long_description entry.

[codecov]

Codecov Report

Merging #1052 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1052   +/-   ##
========================================
  Coverage    97.63%   97.63%           
========================================
  Files          237      237           
  Lines        12371    12371           
========================================
  Hits         12078    12078           
  Misses         293      293           

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