Arena-py

Changed
- MQTT broker connection uses encrypted TLS.

Changed
- Remove default color from `Object`s.

Added
- `Object`s start with a default color of `(128,128,128)`.
- Add first draft of arena-py CLI (similar to mosquitto_pub/sub)!
Usage: `python3 -m arena -s <scene> -a <pub/sub> ...`
Type `python3 -m arena -h` for options.

Changed
- Do not accept None for host, realm, scene in `Scene` constructor.

Added
- Programs will now disconnect and exit on an exception.
- Added try except block on message parsing.
- `WindowsSelectorEventLoopPolicy` added for Windows Python >=3.8

Changed
- `add_msg_callback` auto subscribes to topic.
- Possibly fix `Color` bug with invalid colors.

Changed
- Remove top-level landmark type.

Added
- Replace with new landmark attribute.