Added 'pearson' to methods list in pandas/core/nanops.py
- [x] ref https://github.com/pandas-dev/pandas/pull/30461#discussion_r362323260
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
