Allow publish action to fail uploading to TestPyPI

[hakonanes]

Description of the change

Wanted to check whether docs were not included in the PyPI upload, so I uploaded the source distribution tagged with 0.8.0 from my machine to TestPyPI. The docs were not included, great. But now the automatic upload from GitHub fails because 0.8.0 already exists on TestPyPI. Not great.

This is (temporarily?) fixed by allowing the publish action to fail uploading to TestPyPI.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [n/a] Clean code style by running black via pre-commit

For reviewers

• [ ] The PR title is short, concise, and will make sense 1 year later.
• [ ] New functions are imported in corresponding __init__.py.
• [ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
• [ ] New contributors are added to release.py, .zenodo.json and .all-contributorsrc with the table regenerated.