Modernize package

[tomicapretto]

• Use pyproject.toml to centralize all the package configuration
• Use setuptools_scm for automatic versioning
• Removes setup.py, setup.cfg, flexitext/version.py, MANIFEST.in, requirements.txt, and requirements-dev.txt.
• Updates .github/workflow/tests.yml to use latest version of all actions.
• Replaces miniconda with regular python setup in .github/workflow/tests.yml.
• Adds token for codecov upload.
• Modernize .github/workflow/publish_pypi.yml.

Closes #17 and #19

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b2cadb7) to head (7b8f664).
Report is 10 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff            @@
##              main       #20   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        23    -1     
  Lines          445       446    +1     
=========================================
+ Hits           445       446    +1     

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