BUG: df.replace with numeric values and str to_replace
- [x] closes #34789
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

We also avoid copies by not calling `self.as_array` and instead moving the mask-finding to the block level.