Updating pyproject.toml with valid python versions

[TinyMarsh]

Description

This PR addresses #105 updating the pyproject.toml file to include the Python versions which were included in the CI workflow in #119 . Specifically, the valid versions that MUSE can now be installed in are 3.8 and 3.9.

Closes #105

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python setup.py build_sphinx

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[dalonsoa]

This QA is facing but because of something to change in #118 , so nothing to do here until that is sorted.

@jamesturner246 , let @TinyMarsh know when you're done with that one so he can rebase.

[jamesturner246]

@TinyMarsh @dalonsoa fixed in #118. Was just commas and whitespace fuss.

[TinyMarsh]

I prematurely rebased. It seems I should wait until #118 is merged before rebasing this branch.

[dalonsoa]

#118 Merged! All yours!

[TinyMarsh]

@dalonsoa Rebased and ready for review.