Implement a status `pending`.

[tobiasraabe]

Fixes problems introduced by #99.

• Resort pending tasks by priorities.
• Make _pytask/live.py more flexible to handle more statuses.

[tobiasraabe]

The executor from concurrent.futures are not reporting accurately: https://github.com/python/cpython/issues/81457. Some people are using an object where processes can write into: https://stackoverflow.com/a/56587872/7523785.