ci: cap gevent for free-threaded Python

[dkropachev]

Summary

Cap gevent below 26.4.0 in the shared dev dependency group.

The integration workflow installs the full dev environment for all matrix jobs, including the 3.14t lanes. After gevent 26.4.0 was published, those lanes started failing during uv sync before tests began.

This keeps CI green by resolving to the last known-good gevent line for 3.14t while upstream free-threading support is still unsettled.

Fixes #815

Issue: #815

Upstream references:

• Gevent in a nogil Python world gevent/gevent#2087
• Document free-threading python support (or lack of) gevent/gevent#2127

Validation

Verified locally:

uv venv /tmp/... --python 3.14t
uv pip install --python /tmp/.../bin/python 'gevent<26.4.0'

Result:

Installed 4 packages
 + gevent==25.9.1
 + greenlet==3.4.0
 + zope-event==6.1
 + zope-interface==8.3

Full integration matrix was not run locally.

[Lorak-mmk]

I don't see how this could possibly be a correct solution.
gevent does not currently support free threading. The new version just makes that explicit.
The older version that you pinned to also doesn't support free threading. What will happen if we use it anyway? No idea. Flakiness / crashes? GIL forced enabled, making our free-threaded pipelines useless for testing no-GIL mode? No idea, but no option here is good.

The correct solution is to not have gevent as a dependency when running in free-threaded mode.