xfail: mismatch between docs and the actual behaviour

[hoefling]

I know this is kinda nitpicky, but for the sake of completeness:

The docs state:

If you want to be more specific as to why the test is failing, you can specify a single exception, or a list of exceptions, in the raises argument.

However, the actual implementation of the xfail marker expects a tuple instead of a list, raising an internal error otherwise. May I suggest to replace list with tuple in the above sentence?

[nicoddemus]

Oh good catch @hoefling. Sure, would you like to open a quick PR with that? Thanks!

[hoefling]

@nicoddemus with pleasure! My first time making a pull request, so please be gentle :-)