io/parsers: ensure decimal is str on PythonParser
- [x] closes #29650
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Also can you add a whatsnew note for v1.0.0? @WillAyd thanks for the feedback! 🎉 

Just pushed the changes you suggested and rebased master, please take another look. ptal @WillAyd @jreback 

Thanks for the feedback! thanks @fsouza  🎉 