Consider single element subscript expr for implicit optional (#5717)
## Summary
Consider single element subscript expr for implicit optional.
On `main`, the cases where there is only a single element in the
subscript
list was giving false positives such as for the following:
```python
typing.Union[None]
typing.Literal[None]
```
## Test Plan
`cargo test`
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>