updates @ packaging-projects.rst

[cassiasamp]

hi everyone, I was following the tutorial for packaging projects and thought that some details might help others.

This commit:

• fixes raised value and configuration errors in pyproject.toml for hatchling;
• makes explicit the creation of a LICENSE file (if I hadn't previous knowledge, it might be confusing);
• adds TestPyPI installation example (package wasn't recognized with current's tutorial command).

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📚 Documentation preview 📚: https://python-packaging-user-guide--1818.org.readthedocs.build/en/1818/