Yolapy

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* Remove calculating `Site.is_published` property,
because it's already in api.

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* Add `Yola.set_site_url` method.

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* Run tests using maintained python versions : 2.7, 3.5, 3.6, 3.7, 3.8
* Add `Yola.complete_wl_task(wl_task_id)`
* Add `Yola.fail_wl_task(wl_task_id, reason)`

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* Add `Yola.get_user_wsites` method.

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* Remove `six` dependency
* Remove `User.save` method
* Add `User.create` classmethod
* Add `Yola.get_sso_url` method
* Fix bug caused by improper call signature to `create_site_import` method on
`Yola` service
* Update `User.__init__` to accept `**kwargs` rather than named arguments. This
makes it consistent with other yolapy models and more flexible in case of
changes to data returned by the service.

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* Remove `Yola.subscribe_to_campaign()` and `Yola.cancel_campaign_subscription`
methods, they aren't allowed to be used externally.
* Add SiteImport model and api for creation and listing
* `Yola.get_sso_create_site_url` and `Yola.get_sso_open_site_url`
now accept optional `locale` arguments which define what language the
returned urls should be generated for