REF/TST: use pytest builtin monkeypatch fixture and remove mock fixture
- [n/a ] xref https://github.com/pandas-dev/pandas/pull/24557#issuecomment-451174496
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n/a ] whatsnew entry
