TYP : DataFrame.(merge, join)  core.reshape.merge.(merge, ...) (easy)
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

looks good. @simonjayhawkins if you can double check here. thanks @arw2019 