add test for new union syntax on generic bounds

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closes#10949

[hauntsaninja]

Great, thank you very much! :-)

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@hauntsaninja i'm not sure why the 3.10 tests are failing because it works when i run the same code locally. i assume 3.9 and below are failing because this new union syntax is only enabled in 3.10, so i moved the test to the newly created check-python310.test

[JelleZijlstra]

This looks like a fixture issue to me. You may need a different one. See https://github.com/python/mypy/blob/master/test-data/unit/README.md#fixtures for an intro to fixtures.

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i can't seem to find a fixture that works, i thought maybe [builtins fixtures/type.pyi] since other tests that use the new union syntax use it but it doesn't seem to make a difference

[sobolevn]

LGTM, thank you! 👍