Add pyproject.toml file for PEP 517 builds

[lpsinger]

This file is now required for building Python packages, in the most general case.

Description

< add a description of any changes here>

Checklist

• All new functions and classes are documented and adhere to Google doc style (3.8.3-3.8.6 of this document)
• Add/update sphinx documentation with any relevant changes.
• Add/update pytest-style tests in /tests, ensuring sufficient code coverage.
• make test runs without errors.
• make lint doesn't give any warnings.
• make format doesn't give any code formatting suggestions.
• make doc runs without errors and generated docs render correctly.
• Check that CI pipeline run on this PR passes all stages.
• Review signoff by at least one developer.

NOTE: If this PR relates to a release, open and reference an issue with the Release checklist template.

[codecov]

Codecov Report

Merging #191 (c5ad467) into master (ff07628) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          15       15           
  Lines        1258     1258           
=======================================
  Hits         1256     1256           
  Misses          2        2           

Flag	Coverage Δ
pytest	99.84% <ø> (ø)

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