Switch to setuptools declarative syntax through setup.cfg

[jdufresne]

Avoids mixing code and configuration. This approach can allow for some
automation and also reduces boilerplate in setup.py.

Available since setuptools 30.3.0 (Dec 8, 2016).

https://setuptools.readthedocs.io/en/latest/setuptools.html#configuring-setup-using-setup-cfg-files

Switch to setuptools declarative syntax through setup.cfg

Contributor checklist

• Provided the tests for the changes. (N/A)
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[codecov]

Codecov Report

Merging #1141 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          37       37           
  Lines        2678     2678           
  Branches      322      322           
=======================================
  Hits         2664     2664           
  Misses          8        8           
  Partials        6        6           

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