required argument 'flush_errors' error with latest version of mypy

Hi @dbader,

`pytest-mypy` seems to be broken with the latest release of `mypy==0.570`, but I did not yet have the time to investigate it further or fix it. It yields the following errors for each file:

```
[...]: in runtest
    res = mypy.main.type_check_only(sources, None, options)
E   TypeError: type_check_only() missing 1 required positional argument: 'flush_errors'
```