Do not invoke setupy.py directly.

[Carreau]

invoking setup.py directly is not the best way to install a package from source. See one of the most recent blog about this:

https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html

This updates the readme to invoke pip, and use python -m pip to make sure that the pip which is invoked is the one from the current Python.

i'm also happy to make it python3 -m pip, but these days python points to python 3 most of the time.

References to other Issues or PRs or Relevant literature

Brief description of what is fixed or changed

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[codecov[bot]]

Codecov Report

Merging #418 (29adc2c) into master (6246354) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #418   +/-   ##
=========================================
  Coverage   98.611%   98.611%           
=========================================
  Files           27        27           
  Lines         3530      3530           
=========================================
  Hits          3481      3481           
  Misses          49        49           

[czgdp1807]

Thanks for the PR.