V2 - Make Selenium optional (only required for one function)

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I am using ebay_rest without Selenium - I manage my own redirects as this is a web application. I would not install Selenium on our live server and it wouldn't work anyway in this configuration. All the Selenium imports in token.py are only used in a single function _get_authorization_code which I don't use (I supply the API object with all refresh token information directly).

Selenium is obviously very useful if you need it, but it is a very significant unnecessary requirement if you don't and a bit of a show-stopper for me.

The actual import is easy to deal with; either move the imports back into the relevant function or wrap in a try/except and throw an error if _get_authorization_code gets called. A user-friendly error ('Install Selenium to use the webdriver-based eBay user token authentication') could be displayed.

The trickier bit I guess is that Selenium is a pip required install; I'd prefer a recommended but not required install? ebay_rest isn't an end-user package; this is a developer package so I figure anyone working with it should be able to read the instructions and install Selenium if required (and add it to their requirements.txt/equivalent if packaging it).

Start in token.py, focusing on the Selenium imports and the _get_authorization_code function. Check how Selenium is declared as an installation requirement, then make it optional while preserving the webdriver-based flow and providing a clear installation error when that function is used.

Written by the indexing model from the issue text.