Update constraint for importlib-metadata

[UsamaSadiq]

Thanks for contributing a pull request!

On Python<3.8, importlib-metadata==2.0.0 causes dependency conflicts because of the pinned constraint python<3.8. There is no breaking change in the importlib-metadata version 2.0.0.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
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[UsamaSadiq]

Hi @gaborbernat, thanks for merging the fix. Could you tell me when the next release is planned for tox? Should I make a PR for releasing a new version as well to get this fix updated on PyPI, if the main release is late in the future?