Remove `Any` type on `_convert_type`

[DominicOram]

Once python/mypy#9773 we can remove the | Any in _convert_type.

Acceptance Criteria

• Any type hint removed here
• Type checkers and unit tests still happy

[stan-dot]

@callumforrester have we moved away from mypy to pyright enough that we can remove | Any without waiting for that mypy issue?

At the moment it does seem that we use mypy in the project

[callumforrester]

Pending #619

[abbiemery]

I don't think this is possible with pythons type system. Where there are cases of unions being passed into _convert_type removing the Any will fail as UnionType is not a type. We can remove the type and just have it as:

def _convert_type(self, typ: Any, no_default: bool = True) -> type:

If we want to clean it up a bit and be explicit about what we are doing (as the Any just flags to disregard any types anyway). Otherwise we can just ignore it.

(@tpoliaw for a sanity check?)

[tpoliaw]

The first line of the method is if typ is NoneType so it can't just be type. I remember unions being a pain so so if we're dealing with the Any case anyway, leaving it just as Any makes sense.