Add backwards compatible types to file generated by __test__.py

[hofbi]

As suggested in #435 (comment) a backwards compatible (< Python 3.9) type hint.

Regardless of merging this PR or not, the current 1.0.0 release is broken for < Python 3.9. Since Python 3.8 is already EOL, this is in theory valid, but I guess there are still a few people using Python 3.8 because of Ubuntu 20.04.

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Changes are visible to end-users: no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[hofbi]

When looking for test cases, I found

rules_py/examples/multi_version/BUILD.bazel

Line 26 in 97aff24

python_version = "3.8.12"

which seems to test with Python 3.8, hence I would expect that this test should have failed.

[mattem]

Thanks 🙌
Yeah the tests never ran on #428 as the author hadn't submitted a PR to rules_py before, and I missed clicking the "Approve Workflow Run" to run the tests, and it seems the settings on this repo allows merging with no CI results. If this is green I can merge this and tag a v1.0.1.