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.