github 'maintainer: timeout' label never added to openmaintainer ports

[Chris Jones]

Hi,

The 72 hour maintainer timeout applies to both openmaintainer and 
non-open maintainer

https://guide.macports.org/chunked/project.update-policies.html

however, unless I am mistaken the github macportsbot never applies the 
'maintainer: timeout' label to openmaintainer PRs.

https://github.com/macports/macports-ports/pulls

I think it would be useful to extend macportsbot to do this for both, as 
then it would be easier to scan the open PRs to pick out PRs, both open 
and closed, that have hit the timeout and thus ready to be potentially 
merged.

cheers Chris