Add compatibility for Postponed Evaluation of Annotations (PEP 563)
Hello,

The code says that it will add compatibility for Postponed Evaluation of Annotations ([PEP 563](https://www.python.org/dev/peps/pep-0563/)) when Python 3.9 is released (which already happened on 2020.10.5). Is there any plan to complete this?

https://github.com/huggingface/transformers/blob/2c2a31ffbcfe03339b1721348781aac4fc05bc5e/src/transformers/hf_argparser.py#L85-L90
Hey! We don't have to do the bandwidth to do it right now, but we'd welcome contributions! Let me tag this as a first good issue, and let me know if you're interested in taking a stab at it!<|||||>I'm glad to help with that, maybe it'll take some time. I never contribute here, I'll try to follow the CONTRIBUTING.md, post progress here and submit PR later, any discussion telling me if I'm doing right would be great. <|||||>According to [discussion here](https://bugs.python.org/issue39442) and solution provided by [Pydantic](https://pydantic-docs.helpmanual.io/usage/postponed_annotations/), we may just call [typing.get_type_hints](https://docs.python.org/3.9/library/typing.html#typing.get_type_hints) on some dataclass to get type of a field instead of relying on `field.type`.

Also, `typing` module is still under development, thus changes notably across different versions of Python. Since Python 3.6 reached its end-of-life last year (https://endoflife.date/python), dropping support for Python 3.6 would be reasonable and make this implementation much easier as well. There seems to be no plan on this (see also #15720).<|||||>I understand; as mentioned in the thread you linked, we're unlikely to drop support for Python 3.6 just yet, but we'll definitely keep in mind that this particular issue would be solved in a simpler manner were we to drop support for it.<|||||>I'm glad to help with that, maybe it'll take some time. Like [sparkanime.com](https://sparkanime.com/)<|||||>@rahimt420 Hello, thanks for the offer! However, I've already submitted #15795 to resolve this issue. It will be great if you could leave any suggestions there!