Pywallhaven

- fixed PEP 508 compliance in setup.cfg
- added Python 3.9 to tox tests

- adopted black for code formatting
- dropped support for Python < 3.8
- removed deprecated methods search() and get_collection()
- fixed regex checks for ratios and resolutions, added support for landscape and portrait ratios
- [v0.7 milestone issues](https://github.com/jtait/pywallhaven/milestone/7?closed=1)

- added Tag.alias_list - returns a list of strings from the Tag's alias, split by commas and stripped of leading/trailing whitespace.
- Tag.category_id now stored as int instead of str
- [v0.6 milestone issues](https://github.com/jtait/pywallhaven/milestone/6?closed=1)

- changed behaviour of _as_class properties on Wallpaper to better handle empty fields. Uploader and Tags fields are empty in searches and collections, and are only populated if using the wallpaper endpoint (get_wallpaper).
- added category_list_as_numeric_string function
- Tag.created_at_datetime and Wallpaper.create_at_datetime are now timezone aware and set to UTC
- [v0.5 milestone issues](https://github.com/jtait/pywallhaven/milestone/5?closed=1)

- removed rate limiting
- changed Wallhaven class to allow sub-classes to override get_endpoint(), which will allow more effective rate-limiting of requests
- removed dependency on urllib3 (except through requests) and [ratelimit](https://pypi.org/project/ratelimit/)
- see issue [7](https://github.com/jtait/pywallhaven/issues/7) for more details.
- fixed usage of seed in get_search_pages
- [v0.4 milestone issues](https://github.com/jtait/pywallhaven/milestone/4?closed=1)

- use the [ratelimit](https://pypi.org/project/ratelimit/) library to limit API requests to 45 per minute, as per [the official docs](https://wallhaven.cc/help/api#limits)
- [v0.3 milestone issues](https://github.com/jtait/pywallhaven/milestone/2?closed=1)