Baseline must be a tuple?

[cbrnr]

I was wondering if forcing the baseline argument to be a tuple is necessary and/or added on purpose in #9210 (https://github.com/mne-tools/mne-python/blob/main/mne/baseline.py#L178). I usually tend to use lists, and this used to work before this change. Now, a tuple is required. @hoechenberger? cf cbrnr/mnelab#381

[hoechenberger]

We always explicitly requested baseline to be a tuple as per the API docs, so I suppose this change you mentioned is just to ensure that the output (in the .baseline attribute) equals the input provided by the user. I think it'd be good to support array-like of length 2 as input and cast to a tuple internally

[cbrnr]

Yes, I'd also prefer list-like with two elements if there is no strong reason to require a tuple. If everyone agrees I can submit a PR.

[agramfort]

Works for me