Exclude GDF channels

[cbrnr]

Fixes #8511. It seems like we neither implemented nor tested this feature. Let's see if existing tests pass, I'm going to add a specific test next.

[agramfort]

@cbrnr feel free to merge and backport

thanks a lot

[cbrnr]

Is there a short description on how to backport specific PRs? I don't remember how I did it because that's been a long time.

[larsoner]

Nowadays I:

1. Squash and merge
2. Locally check out maint/0.21 up to date and tracking upstream branch
3. Cherry pick the squashed commit
4. Git mergetool to deal with the deleted file and any conflicts (usually just means hit "d" to delete latest.inc that doesn't exist on maint)
5. Manually copy-paste the change log line to 0.21.inc
6. Git add 0.21.inc
7. Git cherry pick --continue
8. Accept the log message
9. Git push

Sorry for the bad formatting, on mobile. But maybe this is enough hints. If not I can flesh out out later

[cbrnr]

Thanks @larsoner, this worked perfectly! Should we add these instructions to our dev docs?

One more question, I've pushed the changes directly to upstream/maint/0.21 - was this correct or should I have used the branch on my fork? In any case, now I still see the "Create PR" message on the upstream MNE repository, how can I get rid of that?

[larsoner]

Yes push to upstream is correct.

No way to get rid of that prompt that I know of, but lately it has gone away on its own, I think even the same day sometimes

[larsoner]

Feel free to add this information to the wiki, it's probably better there than the dev docs. Only maintainers really need to care about this, similar to our release instructions (which also live in the wiki)

[drammock]

There's been talk of an advanced dev docs page getting added but I think I agree with @larsoner that backporting is rare enough that the wiki is the best place for it.

[cbrnr]

Thanks, I've added a page to the wiki: https://github.com/mne-tools/mne-python/wiki/How-to-backport-a-change