Updated the use of `Type | None` to `Optional[Type]` for support `[email protected]`

[vincentzed]

Motivation

So Python 3.8 is supported. The versions here, they are not tested like this on the regular CI, so it can be missed but we should be careful. I don't think, the version of dependencies here necessarily stops this code path from running.

Modifications

https://discuss.python.org/t/clarification-for-pep-604-is-foo-int-none-to-replace-all-use-of-foo-optional-int/26945/2 All of the replacements are safe.

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