TYP: Enable mypy stricter typing for partial types

[simonjayhawkins]

follow up to #62327

The mypy setting local_partial_types restricts where mypy is allowed to infer a "partial type" from incomplete initialization. With this option enabled mypy will permit partial types only for variables that are local to functions. Module level and class attribute initializations that would otherwise produce a partial type become errors unless you provide an explicit annotation. This forces you to add annotations where mypy otherwise would have silently inferred a partly-known type.

[mroeschke]

Thanks @simonjayhawkins