Flickr-photos-api

* Remember to expose the `UserInfo` type at the top level.
Now you can import it as `from flickr_photos_api import UserInfo`.

* Return more information from the `lookup_user_by_id()` and `lookup_user_by_url()` methods.

In particular, they now return a new type `UserInfo` which includes three new fields:

- `count_photos` (int)
- `description` (str or None)
- `has_pro_account` (bool)

* Retry errors with code 201 from the Flickr API, which usually indicates a transient issue rather than a permanent failure.
This should make using the API slightly more reliable.

* Fix a bug where looking up videos could throw a ValueError.

* Fix a bug where some photos would be returned with location information, even though the location accuracy is `0`, which means it's so vague as to be unusable.
* Reshuffle some of the internal utility methods to make them slightly easier to reuse in downstream code.

* Add a new exception `InvalidXmlException` which is thrown when the Flickr API returns a response which isn't valid XML. The error will be retried up to three times in case it's a transient error, and if not, the offending XML is included in the error message.