Make package PEP 561 compatible
Pandas has growing number of type annotations but these are not properly advertised to type checking tools. According to [PEP 561](https://www.python.org/dev/peps/pep-0561/) packages providing annotations should contain `py.typed` files in corresponding packages.

`py.typed` is applied recursively, if placed in the root, and can accept missing annotations, if contains `partial\n`.

This PR adds such file and includes it in `package_data`.

- [x] closes #28142
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
