matplotlib 3.8 + minimum dependencies check

[AdamOrmondroyd]

As usual, matplotlib 3.8 has broken several things in anesthetic. As the necessary changes aren't straightforward (see #339 ), I was going to restrict the version to

dependencies = [
    
    "matplotlib>=3.6.1,<3.8.0",
    
]

However, this doesn't work with the minimum dependencies test, as it is nonsense to have == and < in the same requirement.

[AdamOrmondroyd]

I've looked into poetry, which has a --use-lowest-versions option we might be able to use for this test instead

Edit - this doesn't exist 😭