Discourage bad practices and remove "use_https" argument from the constructor
all together and update affected tests.

If user wants to use a custom non-https URL, they can still do that by passing
api_urls argument to the constructor.
Update CHANGES.
Throw if api_urls argument contains an invalid URL.
eplace CA_CERTS_BUNDLE_PATH module level variable with a "ca_certs_bundle_path"
argument which can be passed to the Yubico class constructor.
Update CHANGES.
Update docs with information on how to use a custom ca bundle.
Remove now unused variable.