gh-135959: Mark ast.ImportFrom.level as required

[JelleZijlstra]

• Issue: ast.ImportFrom.level is incorrectly marked as optional #135959

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[hunterhogan]

So, a pull request to fix the certainly incorrect code is in limbo because it is failing the automated tests. It is failing the automated tests because the tests are not testing for the correct responses: they are testing for the incorrect responses that were hardcoded into the tests by humans who made a tiny error in a massive codebase. Is this unittest-22? You can't change the code if it doesn't pass the unittest, and you can't pass the unittest because you can't change the code.