Make docopt exceptions public

[sodul]

There are use cases for capturing docopt exceptions such as DocoptExit. Unfortunately since docopt.py hides them through the __all__ statement this makes them effectively private.

The types-docopt package would cause mypy to error when using docopt.DocoptExit while doing exception handling in client code. This was fixed in python/typeshed#9839 but I believe docopt itself should declare these as public.