fix: Add build-system to pyproject.toml

[kratsg]

Description

Fixes #638

This PR needs to add [build-system] in pyproject.toml which was missing in order to run the pypa-publishing recipe introduced in #638. Additionally, we didn't catch this because the test workflow wasn't being run to check if we could build... now we check and we're good to go.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Fixes PR #638
* Add build-system in pyproject.toml which was missing in order to run the PyPA-publishing recipe introduced in #638
* Make sure workflow for publish builds packages, but does not publish to TestPyPI

[codecov]

Codecov Report

Merging #639 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #639   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files          42       42           
  Lines        2547     2547           
  Branches      345      345           
=======================================
  Hits         2418     2418           
  Misses         85       85           
  Partials       44       44

Flag	Coverage Δ
#unittests	94.93% <ø> (ø)	⬆️

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[matthewfeickert]

LGTM. Thanks for catching this, @kratsg!