bpo-36567: use debian's manpage service for references

[isidentical]

use MANPAGES_URL env variable for manpage references. If that variable not found, use debians manpage service.

https://bugs.python.org/issue36567

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

And if you don't make the requested changes, you will be put in the comfy chair!

[isidentical]

I have made the requested changes; please review again

[bedevere-bot]

Thanks for making the requested changes!

@berkerpeksag: please review the changes made to this pull request.

[berkerpeksag]

LGTM! I just tested this branch locally and it worked as expected.

[miss-islington]

Thanks @isidentical for the PR, and @berkerpeksag for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Thanks @isidentical for the PR, and @berkerpeksag for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Sorry, @isidentical and @berkerpeksag, I could not cleanly backport this to 3.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker eab99650799699f766c2660f4cfa8ff3f9e8457f 3.7

[berkerpeksag]

Ok, let's not bother backporting to 3.7 then.