Fix Context.cd() behavior to match the type hint

[rouge8]

Issue found when fixing the type hint in typeshed: python/typeshed#9823 (review)

[rouge8]

Ahh I see, reading the docstring it's the typehint that is wrong. Will fix.

[rouge8]

Hmm, so the way I see to fix this would be to use a typing.Protocol with a __str__ method, but it looks like invoke supports Python 3.6 which doesn't have that and doesn't currently have a dependency on typing_extensions. Should I add a dependency on typing_extensions to the dev requirements?

[JelleZijlstra]

All objects have __str__ so a protocol for __str__ doesn't make sense. If you only want to support objects that are actually meant to be paths, os.fspath is a better choice.

[kuwv]

I should have checked typeshed. I knew there were types for paramiko but no idea one was created for invoke. I'll see what I can reuse here.

Thanks for the assist!

[kuwv]

#956 The os.fspath requires an object to have __fspath__ instead of __str__ for the tests to pass.