Luminoso-api

* Updated the retry adapter to retry nine times instead of once (resulting in
a four and a half minute wait before the last retry). While a single retry
after one second is sufficient for most uses, some users reported unhandled
429 responses when using multiple threads.

Please note that we do not recommend using multiple threads for API calls.
As the rate limit applies to all calls collectively and not separately to
each thread, parallel calls will not be any faster.

* Added an adapter to the client to retry after one second upon receiving a
429 error. (At the time of release, the API does not yet return 429
errors, but we are planning the addition of rate limiting: still allowing
multiple calls per second, but only up to a limit to be determined. A
one-second delay in retrying will suffice and should not impact most use
cases.)

* Fixed the `lumi-upload` command to use the correct (i.e., non-deprecated)
parameter for specifying a workspace.

* Removed v4 client. Now that all v4 API functionality exists in the v5 API,
new users should never need the old v4 client. Existing users who have not
yet transitioned their code to use v5 should continue to use version 2.2.0.

* Removed other long-deprecated functionality:

- Compatibility with Python 2

- Ability to connect to the v5 API with a password, which was introduced
in version 1.1.1 to ease the transition from v4 to v5; connecting to the
API should now be done entirely through a stored token, retrieved either
through the UI or through the v5 client's save_token() method, which does
allow password authentication for that purpose

- Requirement for ftfy in setup (which hasn't been needed for several
years)

* Removed `save_token()` from the v4 client, as the API endpoints that
supported it have been removed.

* Removed the ability to specify a token when using lumi-api, lumi-download,
and lumi-upload.

* Added a new command, `lumi-save-token`, as an interface to LuminosoClient's
existing "save_token" functionality.

* Added the option to have the LuminosoClient create a new API token when
using the `save_token()` method, rather than requiring a token acquired
through the UI.

* Tweaked the `lumi-download` command to improve performance by using the
(relatively new) `fields` parameter on the document-downloading endpoint.