revert misuse of programmatical approach in setup.py

[FCamborda]

setup.py was ditched a long time ago and newer PEPs recommend using setup.cfg or pyproject.toml

The last changes in setup.py created a misdirected dependency between requirements.txt and setup.py, which included an optional dependency that is causing problems upstreams.

This PR merely reverts those changes in favour for the old already-working setup.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1045f57) 34.49% compared to head (21993fd) 34.49%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1303   +/-   ##
=======================================
  Coverage   34.49%   34.49%           
=======================================
  Files          45       45           
  Lines        8257     8257           
  Branches     1146     1146           
=======================================
  Hits         2848     2848           
  Misses       5295     5295           
  Partials      114      114           

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