Convert to more specific return types in DICOMwebClient

[ntenenz]

While it's common practice to accept Sequence[T] when a function can accept a list, tuple, etc., it's often best to be more specific with return types allowing those invoking the methods to have greater clarity of the type returned. In many of DICOMwebClient's methods, current type hints provide a return value of Sequence[T] where List[T] would be more specific. I propose updating these types to the more specific values provided the API is comfortable committing to these types (which seems like a fairly innocuous commitment).

[hackermd]

Good point. List[T is also what the corresponding docstrings say.

[hackermd]

Merged into master branch. Will be included in next release.