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* To discourage bad practices, remove ``use_https`` argument from the `Yubico`
class constructor all together. Also update ``DEFAULT_API_URLS`` variable to
contain full URLs with a scheme (e.g.
``https://api.yubico.com/wsapi/2.0/verify``).
If a user wants to use a custom non-https URL or URLs, they can still do that
by passing ``api_urls`` argument with custom non-https URLs to the
constructor.
* Replace ``CA_CERTS_BUNDLE_PATH`` module level variable with a
``ca_certs_bundle_path`` argument which can be passed to the Yubico class
constructor.
* Update ``requests`` dependency from ``1.2`` to ``2.2``.