The `py.typed` file is marker that signals to type checkers that the
package has type annotations. This is required for tools like mypy to
make use of the type annotations.

Type checkers like `mypy` do not actually run the code they are checking.
This makes certain legal constructs problematic for them.

In this case, constructing the `namedtuple` from the keys of a
dictionary makes it impossible for the type checker to know the names of
the values of the tuple.

Instead, we use a `typing.NamedTuple` and list the fields manually. With
this change, the type checker knows the names of the fields and their
expected type.