DEP: Add 'python_requires' to setup.py to drop 3.4 support
- [x] closes #20697 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This is a small change to the setup.py that *should* stop newer versions of pandas from being downloaded in environments with older/unsupported versions of python. I followed the instructions here: https://packaging.python.org/guides/dropping-older-python-versions/