Python issue with import from typing

Python's **TextIO** used to be in **typing.io** so you would do:
```python
from typing.io import TextIO
```
Since Python 3.6, **TextIO** is now in **typing** directly so the above (which ANTLR does) would trigger an error, it should be changed to [for 3.6+]:
```python
from typing import TextIO
```
(also the underlying reason for https://github.com/antlr/antlr4/issues/2193)