Rlapi

Bug fixes:
- fetching stats will no longer fail when API responds with some `null`s

Enhancements:
- added `py.typed` for proper type hinting support

Breaking changes:
- `reward_ready` attribute of `rlapi.SeasonRewards` is now called `can_advance`

Enhancements:
- Python 3.8 support

Dependency changes:
- removed `defusedxml`
- minimum version of `lxml` is now `4.4.2`
- added `typing_extensions==3.7.4` (Python 3.7 only)

Misc:
- docs improvements
- we're using black code style now
- `rlapi` is now typed

Initial release.