[#10200] Force using GTK+3 in twisted.internet.gireactor

[mgorny]

Scope and purpose

Fixes #10200

Twisted uses incompatible GTK+4 when installed. This patch forces GTK+3 to avoid the problems.

It does not cover adding GTK+4 compatibility ;-).

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10200
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• The title of the PR starts with the associated Trac ticket number (without the # character).
• I have updated the automated tests and checked that all checks for the PR are green.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.
• The merge commit will use the below format
  The first line is automatically generated by GitHub based on PR ID and branch name.
  The other lines generated by GitHub should be replaced.

Merge pull request #123 from twisted/4356-branch-name-with-trac-id

Author: <github_username>, <github_usernames_if_more_authors>
Reviewer: <github_username>, <github_username_if_more_reviewers>
Fixes: ticket:<trac_ticket_number>, ticket:<another_if_more_in_one_PR>

Long description providing a summary of these changes.
(as long as you wish)

[mgorny]

I don't think I can improve the coverage on this, really (or at least not within the scope of this PR).

[glyph]

I don't think I can improve the coverage on this, really (or at least not within the scope of this PR).

It's likely that, particularly with an overflowing review queue, reviewers are likely to skip over this unless it has a passing commit status. If you don't think that coverage on these lines are important, could you put # pragma: no-cover lines to make codecov chill out? The reviewer can then see it's intentional and potentially disagree, but the checkmark will be green at least :). (And you can include a comment next to it explaining why coverage is difficult.)

[mgorny]

I don't think I can improve the coverage on this, really (or at least not within the scope of this PR).

It's likely that, particularly with an overflowing review queue, reviewers are likely to skip over this unless it has a passing commit status. If you don't think that coverage on these lines are important, could you put # pragma: no-cover lines to make codecov chill out? The reviewer can then see it's intentional and potentially disagree, but the checkmark will be green at least :). (And you can include a comment next to it explaining why coverage is difficult.)

Thanks for the suggestion. However, no matter how hard I try, it doesn't seem to work :-(. Maybe the no cover pragma doesn't work for diffs?

[adiroiban]

I am updating the branch.

We now run some GTK reactors tests as part of the CI.

[adiroiban]

I think that this needs-changes

See #10200 (comment) comment from here

There is a report that the code still needs changes

[glyph]

I'm working on a fix in #11706 that somewhat obviates the need for this; there is now only one place we import Gtk, and it's in the test suite, which can be controlled with an env var if so desired, but defaulting to 4.0 (which, hey, has actual compatibility guarantees, supposedly!).

We will need some updates in the docs, though, since we still refer to truly ancient gtk2-style code in a lot of places.

[glyph]

I think that #11705 fixed the underlying problem here. If Gtk3 systems still need help, please open a new PR.