BUG: use cmath to test complex number equality in pandas._testing
- [x] closes #28235
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Adds `cmath`-based equality testing for complex numeric types (`complex`, `np.complex64` and `np.complex128`) 