Add pyproject.toml

[jdebacker]

This PR adds a pyproject.toml file to avoid warnings and eventual deprecation of the ability build an editable package with pip install -e . (see Issue #11457 in the pypa/pip repo).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (b3bca61) to head (cd9b98d).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2840   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          13       13           
  Lines        2617     2617           
=======================================
  Hits         2602     2602           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <ø> (ø)

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