add importlib_metadata as dependency as it is required for py<3.8
Author: cdeepali
Pull request: yes

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Fixes https://github.com/huggingface/transformers/issues/11399


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Comments:
Shouldn't this only be installed for python versions inferior to 3.8?

@LysandreJik, thanks for reviewing my PR.

I think at the moment, we cannot use selectors with `noarch` python packages. Please see warning at - https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html?highlight=preprocess-selectors#architecture-independent-packages. 

I see! Let's go with it then. Thanks for the fix!

Could you open a PR against the `master` branch so that we can also apply that fix for future conda releases?

Also, are you savvy about `conda`? We're having an issue with our recent version releases: [failing suite](https://github.com/huggingface/transformers/runs/2334821335?check_suite_focus=true), do you have an idea of what might be the conflict happening here?

Thanks @LysandreJik for merging this one. Yes I will open a PR against master too. 

Added PR https://github.com/huggingface/transformers/pull/11591 for master branch.

> Also, are you savvy about `conda`? We're having an issue with our recent version releases: [failing suite](https://github.com/huggingface/transformers/runs/2334821335?check_suite_focus=true), do you have an idea of what might be the conflict happening here?

I think this is failing because `python` version in the build env is `3.9` and we do not have `tokenizers` for `py39` on `HuggingFace` channel. 
```
$ conda search tokenizers=0.10.2 -c HuggingFace
Loading channels: done
# Name                       Version           Build  Channel
tokenizers                    0.10.2          py35_0  HuggingFace
tokenizers                    0.10.2          py36_0  HuggingFace
tokenizers                    0.10.2          py37_0  HuggingFace
tokenizers                    0.10.2          py38_0  HuggingFace
```
Looks like anaconda upload failed for tokenizers 3.9 - https://github.com/huggingface/tokenizers/runs/2272898351#step:8:17

I was able to build transformers locally with py38. 

Ah, I thought we were already building on 3.8, that's my bad. Thanks for your help!