mypy

We aren't checking any types that are imported from external libraries but it would be good if we did. I think the change we need to make is in [mypy.ini](https://github.com/alan-turing-institute/sqlsynthgen/blob/a033aabfb522220f669e16b3948f032aa095a56e/mypy.ini#L12) where we probably need to change to

```text
ignore_missing_imports = False
```

There may be other checks that we want to make stricter too.

Review mypy.ini around line 12 and inspect the current mypy configuration. Run the project's mypy checks with external-import checking enabled, then assess any resulting errors and the other checks that should be stricter. Done means the intended stricter type checking is configured and the resulting checks pass.

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