API: add dtype= option to python parser
- [x] part of #12686
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Ultimately I'm working towards #8212 (types in excel parser), which should be pretty straightforward after this.

Right now the tests are moved from `c_parser_only.py`, may need to add some more
cc @gfyoung 
