Replace pkg_resources with importlib_metadata
# What does this PR do?

Fixes #10964. The other reason for this change is that pkg_resources has been [deprecated](https://github.com/pypa/setuptools/commit/8fe85c22cee7fde5e6af571b30f864bad156a010) in favor of importlib_metadata.

`ImportError` looked like the best replacement for `pkg_resources.VersionConflict` since importlib has no equivalent exception, but I'm happy to change it to better suggestions.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. #10964
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation). n/a
- [x] Did you write any new necessary tests? n/a


## Who can review?

CC @stas00

This is odd, CI is not being triggered 2nd time in a row!

I pushed an empty commit and now it started the CI. 