ssl.Purpose.CLIENT_AUTH is incompatible with ssl.Purpose

I'm using Python 3.7.2 on Linux x64, mypy 0.660. This code doesn't pass mypy check:
```python3
import ssl

def t(x: ssl.Purpose):
    pass

t(ssl.Purpose.CLIENT_AUTH)
```
CLIENT_AUTH should be compatible with ssl.Purpose:
```python3
>>> import ssl
>>> isinstance(ssl.Purpose.CLIENT_AUTH, ssl.Purpose)
True
```