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