Fix bug in function validate_type for Python >= 3.9
_The documentation is not available anymore as the PR was closed or merged._
Fix `validate_type` function, so that it uses `get_origin` instead. This makes the function forward compatible.

This fixes #4811 because:
```python
In [4]: typing.Optional[str]
Out[4]: typing.Optional[str]

In [5]: get_origin(typing.Optional[str])
Out[5]: typing.Union
```

Fix #4811.