CLN: rename lib.isscalar to lib.is_scalar
- [x] closes #18987
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

There's also a handful of methods which call `is_scalar` that are named `isscalar`, e.g.:
https://github.com/pandas-dev/pandas/blob/84335621ad0a0d83302a80b6911d3985c00b5cee/pandas/core/computation/ops.py#L101-L103
should these be changed as well?

Also does a CLN require an entry in whatsnew?

