API/COMPAT: support axis=None for logical reduction (reduce over all axes)
- [x] closes #19976
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This is the minimal fix, just to get np.all / np.any working again.

Some followup items:

1. Expand to all aggregations, not just logical ones
2. Do logical reductions blockwiwse: https://github.com/pandas-dev/pandas/issues/17667. Currently, we do `DataFrame.values`, which isn't necessary for logical reductions.