Update in preparation for Python3.8 deprecation of importing ABCs from collections

[znicholls]

Updates the use of collections in preparation for Python3.8, see python/cpython@c66f9f8 (and also avoids deprecation warnings when using iris in Python3.7).

[znicholls]

Test failures appear to all be plotting and docs related so I think this is blocked for now

[znicholls]

@DPeterK is my understanding correct that this is failing for reasons other than the changes made in the PR?

[rcomer]

@znicholls you are right, all recent PRs against master have these errors. I believe these should be fixed once #3316 is merged, then we can all rebase!

[znicholls]

@rcomer are you the right person to ask for a review?

[rcomer]

are you the right person to ask for a review?

@bjlittle will know the best person to nominate to review this.

[znicholls]

@bjlittle are you able to review this one, I think it's pretty trivial?

[ajdawson]

This looks OK except for the change to collections.Counter which I believe is incorrect. Can you sort that out @znicholls and we can get this merged.

[znicholls]

Should be sorted now cheers @ajdawson !

[ajdawson]

Thanks @znicholls, would you mind squashing into a single commit? In case you don't know, you can squash these commits by doing an interactive rebase onto the commit before your first one git rebase -i b209642 and choose squash/fixup for every commit except the first. Let me know if you have questions.

[znicholls]

Assuming I haven't mucked up the header handling I think this should be good now

[pp-mo]

would you mind squashing into a single commit?

Just intrigued @ajdawson - why do you want @znicholls to do that, when you can squash on merge yourself : is it just for the contributor to write the commit message ?

[ajdawson]

Just intrigued @ajdawson - why do you want @znicholls to do that, when you can squash on merge yourself : is it just for the contributor to write the commit message ?

Habit I suppose... I assume you are referring to the squash-and-merge button? In this case that would have been a sensible use of it.

[pp-mo]

@ajdawson Habit I suppose...

😁