Upload s2downloader to PyPi

PyPi:

• add pypi upload to ci.yml (see arosics)
• setup.py: classifiers, python version, project_urls, keywords (see arosics)
• MANIFEST.in to exclude non necessary files (see arosics)

run offline to check *tar.gz like:

# create .tar.gz
python -m build --sdist
# check with twine
twine check dist/*

• add twine variables to GitLab docu
• test on test.pypi.org

condaforge:

• fork staged-recipes repo
• grayskull to create meta.yaml from local repo automatically
• change maintainers if needed, entry points
• add to staged-recipes/recipe/projectname/meta.yaml, open pull request
• wait for feedstock repo