[3.9] bpo-46345: Add a test case for implicit Optional class attribute (GH-30535)

[miss-islington]

(cherry picked from commit 1de6015)

Co-authored-by: Nikita Sobolev mail@sobolevn.me

https://bugs.python.org/issue46345

[miss-islington]

@sobolevn and @gvanrossum: Status check is done, and it's a failure ❌ .

[gvanrossum]

Let's see if the tests pass before approving this. (The other PR was backported this far, right?)

…

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[miss-islington]

@sobolevn and @gvanrossum: Status check is done, and it's a success ✅ .

[corona10]

@gvanrossum cc @sobolevn

When I take a look at previous PRs, most of adding test cases were not backported.
The backported cases were triggered by bug issues.
But let's pass this case as an exceptional case.