Make lupa and aioredis optional dependencies

[cunla]

See discussion on #13. lupa and aioredis should be optional dependencies.

• Make lupa and aioredis optional in pyproject.toml
• Create a test scheme where lupa is not installed.

I see that in #12 you migrated to poetry, where before you did have lupa and aioredis as optional in your setup.cfg:

[options.extras_require]
lua =
    lupa
aioredis =
    aioredis

but it doesn't seem to be the case in the pyproject.toml today with Poetry, unless there is some different way that I'm not aware of to mark them as optional. As per the poetry documentation that's how extras are 'expected' to be defined at least - if there's a different way it's specified in this project then I'd love to learn about that.