[BUG] [flytekit] Support typing.Tuple for multiple returns

[wild-endeavor]

Describe the bug
Currently, if a task returns two or more outputs, users are expected to declare those return signatures as

def t1() -> (int, int):
  ...

This is fine but is not idiomatic and doesn't pass mypy. The correct thing to support is:

def t1() -> typing.Tuple[int, int]:
  ...