F py38

Summary

For #32 (closed). Makes type hinting compatible with Python<3.9

Focus

Simply replace non-subscriptable types by the corresponding typing classes.

Test Environment

Run tox on a machine with Python3.8 installed. You may have to remove the .tox directory beforehand to remove old yaml-header-tools installations.

Check List for the Author

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
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• Check: Are there specifications? Are they satisfied?

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