Fix mypy errors in shuttle.py

[tianyizheng02]

Run mypy with --strict flag and fix resulting mypy errors in shuttle.py

Contributes to #45 by fixing existing type hint errors prior to introducing mypy to the repo.

Before:

> mypy --strict pittapi/shuttle.py
pittapi/shuttle.py:37: error: Returning Any from function declared to return "dict[str, Any]"  [no-any-return]
pittapi/shuttle.py:43: error: Argument "params" to "get" of "Session" has incompatible type "dict[str, object]"; expected "SupportsItems[str | bytes | int | float, str | bytes | int | float | Iterable[str | bytes | int | float] | None] | tuple[str | bytes | int | float, str | bytes | int | float | Iterable[str | bytes | int | float] | None] | Iterable[tuple[str | bytes | int | float, str | bytes | int | float | Iterable[str | bytes | int | float] | None]] | str | bytes | None"  [arg-type]
pittapi/shuttle.py:44: error: Returning Any from function declared to return "dict[str, Any]"  [no-any-return]
pittapi/shuttle.py:51: error: Returning Any from function declared to return "dict[str, Any]"  [no-any-return]
pittapi/shuttle.py:58: error: Returning Any from function declared to return "dict[str, Any]"  [no-any-return]
Found 5 errors in 1 file (checked 1 source file)

After:

> mypy --strict pittapi/shuttle.py        
Success: no issues found in 1 source file