Add is_async switch to new_client_from_enviroment

[mjpieters]

This change makes it possible to use new_client_from_enviroment() and still
select between async and sync clients based on the current codebase.

The change includes the type hints needed for type checkers to know what kind
of object is returned in what circumstances; e.g.

# client: onepasswordconnectsdk.client.Client
client = new_client_from_environment(is_async=False)

# client: onepasswordconnectsdk.async_client.AsyncClient
async_client = new_client_from_environment(is_async=True)

This makes working with the library a lot easier in IDEs with type checker
support as they now know what exact methods a client supports.

This fixes #84

[mjpieters]

typing-extensions 1.4.8 drops support for Python 3.7, so the version is pinned.

[mjpieters]

typing.Literal was added in Python 3.8, which is why we need the backported version here.

If this project were to drop support for 3.7, the import could be updated.

[mjpieters]

Happy to drop this PR if / when #92 is merged.