Drop support for Python 3.8

[MarkZH]

On May 1, 2023, lichess-bot officially drops support for Python 3.8 and will require at least Python 3.9 going forward. The main benefit will be to finish type hinting with the Queue type. Further changes:

• Use the MutableSequence type for MULTIPROCESSING_LIST_TYPE that describes the challenge_queue. This is slightly more generic than the specific type returned by multiprocessing.Manager().list(), but MutableSequence is still an accurate description of what it is. Functions that only read the challenge_queue can use a Sequence type to flag modifications as an error.
• Put all packages for running tests in requirements.txt and delete the separate pip lines from the github actions.
• Create a function that sets restart to False in lichess-bot.py when testing. Functions seem to be more visible to mypy than variables.

[MarkZH]

This is ready to merge.