publish package to PyPI

[cpodalak]

s2dm is a new package that hasn't been published yet, we need to:

1. Create a PyPI account
2. Generate an "Entire account" scoped token initially
3. Add it to GitHub secrets as PYPI_API_TOKEN
4. After first publish, consider creating a project-specific token for s2dm

Publishing to PyPI will make s2dm installable via pip install s2dm

[cpodalak]

After PYPI_API_TOKEN is added to secret, review and test publish job.

#137