This change adds an implementation of the Device Authorization flow
client from RFC8628.  The initial structure is derived from the
existing BackendApplicationClient with the addition of the device_code
in the client.

This change does not provide the support necessary for querying the
device code endpoint in order to generate the initial device_code and
URL that is required for completing the full end to end device
authorization process.

In order to perform the full device authorization flow it's necessary
to first generate the device code and get the authorization flow URL.

prepare_request_uri() allows us to do this while providing scopes and
additional parameters.

These lines are not required for python3