ci: Remove socksio from requirements-testing.txt (#6617)

Remove the package from `requirements-testing.txt` and add it only as a dependency of the test suites that rely on it.
This is part of environment cleanup before moving the remaining testing dependencies to a uv dependency group.

Author: alexander-alderman-webb

requirements-testing.txt

@@ -7,7 +7,6 @@ jsonschema
 executing
 asttokens
 responses
-socksio
 httpcore[http2]
 setuptools
 docker

scripts/populate_tox/tox.jinja

@@ -83,6 +83,7 @@ deps =
     common: pysocks
     common: pytest-asyncio
     common: httpcore[asyncio]
+    common: socksio
     # See https://github.com/pytest-dev/pytest/issues/9621
     # and https://github.com/pytest-dev/pytest-forked/issues/67
     # for justification of the upper bound on pytest
@@ -103,6 +104,7 @@ deps =
     gevent: pysocks
     gevent: pytest-asyncio
     gevent: setuptools<82
+    gevent: socksio
     {py3.10,py3.11}-gevent: zope.event<5.0.0
     {py3.10,py3.11}-gevent: zope.interface<8.0