Support 'reason' argument to `pytest.fail` (#5040)
## Summary
Per the [API
reference](https://docs.pytest.org/en/7.1.x/reference/reference.html#pytest.fail),
`reason` was added in version 7, and is equivalent to `msg` (but
preferred going forward).
I also grepped for `msg` usages in `flake8_pytest_style`, but found no
others (apart from those that reference `unittest` APIs.)
Closes #3387.
