Changed pypi tag of Cython
pip install cython -> pip install Cython

- [x] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

both versions work, why should we prefer the uppercase one? Feels like hacktoberfest stuff, closing.