Add Support for Python 3.14+

[rmartin16]

What is the problem or limitation you are having?

Python 3.12 deprecated AbstractChildWatcher as captured in #76 and it has been removed in Python 3.14. The plan seemed to be to use PidfdChildWatcher once support for Python 3.8 was dropped....however, PidfdChildWatcher has been removed from Python 3.14 as well. In general, it appears the whole concept of "child watchers" was removed from Python.

Therefore, Python 3.14 and beyond is not currently supported.

Describe the solution you'd like

Add support for Python 3.14+.

Changes in #137 already skip imports for GLibChildWatcher for 3.14+ and with a few additional hacks, I was able to get gbulb to work but the tests still fail.

Describe alternatives you've considered

n/a

Additional context

With the availability of Python 3.14.0a0, I was curious about the current level of support. This was the first blocker I ran in to for building a Briefcase app.

[freakboy3742]

Definitely something we would need to address for Python 3.14 support - but FWIW, It's entirely likely that gbulb as a project will be deprecated very soon. As noted in #32, the next release of PyGObject will have support for native asyncio integration, and with that, there's no need for gbulb.

My initial experimentation is that this new native support isn't subject to #58 and #73, and requires almost no code changes to Toga. See beeware/toga#2550 for a prototype patch. I'd be very surprised if we don't adopt that, rather than trying to fix gbulb for 3.14 support.