explicitly specify build-backend

[pastalian]

setuptools.build_meta:__legacy__ will be used if build-backend is not specified. Use setuptools.build_meta instead.

https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#fallback-behaviour https://pydocbrowser.github.io/setuptools/latest/setuptools.build_meta._BuildMetaLegacyBackend.html

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.88%. Comparing base (64680d5) to head (1fa2a04). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #162 +/- ## ======================================= Coverage 88.88% 88.88% ======================================= Files 3 3 Lines 360 360 ======================================= Hits 320 320 Misses 40 40 ```

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