TextIOWrapper.encoding missing Optional Argument

Happy to submit a PR to fix but I think this is wrong

```python
from io import BytesIO, TextIOWrapper
from typing import Optional

encoding = None  # type: Optional[str]
buf = BytesIO()
TextIOWrapper(buf, encoding=encoding)
```

```sh
mypy_test.py:6: error: Argument "encoding" to "TextIOWrapper" has incompatible type "Optional[str]"; expected "str"
```