Description

This PR fixes some pyright errors:

Make _gen_grid abstract in MiniGridEnv instead of assert False
Raise an error instead or a string in roomgrid.py
For some reason, pyright doesn't like gym.Wrapper or gym.ObservationWrapper. I changed the corresponding lines to import from gym.core instead.
In run_tests.py, EmptyEnv5x5 doesn't exist anymore, and the list of registered Minigrid environments should be accessed from gym's registry.
In FlatObsWrapper, when a mission contained ,, an error was raised. This should be fixed.

Type of change

Please delete options that are not relevant.

[x] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes