Type hint compatibility error with `typing.get_type_hints()` on Python 3.8 and 3.9

[vdusek]

Remove the fallback in https://github.com/apify/apify-sdk-python/blob/v1.4.0/tests/unit/conftest.py#L86:L104 once we get rid of support for Python 3.8 and Python 3.9.