PEP 570 (Positional-only Argument Syntax) tracker

This issue is supposed to track when we can start to use Python's standard positional-only argument syntax ([PEP 570](https://www.python.org/dev/peps/pep-0570/)) in typeshed. Support is needed in released versions of:

* [x] mypy (supported when running under Python 3.8+)
* [x] pytype
* [x] pyright
* [x] pyre
* [X] PyCharm