Expose typing hints as they are part of the API

[eccles]

Just recently upgraded to 2.2.1 from 1.11 and pyright gave me lots of type hint errors.

/home/builder/archivist/confirmer.py
  /home/builder/archivist/confirmer.py:84:15 - error: Argument of type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to parameter "on_giveup" of type "_Handler | Iterable[_Handler]" in function "on_predicate"
    Type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to type "_Handler | Iterable[_Handler]"
      Type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to type "_Handler"
        Parameter 1: type "Details" cannot be assigned to type "dict[str, Any]"
          "Details" is incompatible with "dict[str, Any]"
      "function" is incompatible with protocol "Iterable[_Handler]"
        "_iter_" is not present (reportGeneralTypeIssues)
  /home/builder/archivist/confirmer.py:122:15 - error: Argument of type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to parameter "on_giveup" of type "_Handler | Iterable[_Handler]" in function "on_predicate"
    Type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to type "_Handler | Iterable[_Handler]"
      Type "(details: dict[str, Any]) -> NoReturn" cannot be assigned to type "_Handler"
        Parameter 1: type "Details" cannot be assigned to type "dict[str, Any]"
          "Details" is incompatible with "dict[str, Any]"
      "function" is incompatible with protocol "Iterable[_Handler]"
        "_iter_" is not present (reportGeneralTypeIssues)

In order to fix this I had to import a private directory viz:

from backoff._typing import Details

Surely type hints are part of the API and should not be private ?