[ty] Don't require default typevars when specializing (#17872)
If a typevar is declared as having a default, we shouldn't require a
type to be specified for that typevar when explicitly specializing a
generic class:
```py
class WithDefault[T, U = int]: ...
reveal_type(WithDefault[str]()) # revealed: WithDefault[str, int]
```
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
