Reformat and add --allow-unix-socket to pytest config

Seems like some underlying change in multiprocessing or some related
module in Python 3.14 now means our test suite uses unix sockets. We
have a pytest plugin installed that blocks sockets by default to avoid
mistakenly adding tests that rely on the Internet. Local unix sockets
are totally fine, of course. I've added them to the allowlist.

Author: ichard26

pyproject.toml

@@ -300,7 +300,15 @@ follow_imports = "skip"
 #
 
 [tool.pytest.ini_options]
-addopts = "--ignore src/pip/_vendor --ignore tests/tests_cache -r aR --color=yes --disable-socket --allow-hosts=localhost"
+addopts = [
+    "-r=aR",
+    "--color=yes",
+    "--ignore=src/pip/_vendor",
+    "--ignore=tests/tests_cache",
+    "--disable-socket",
+    "--allow-unix-socket",
+    "--allow-hosts=localhost",
+]
 xfail_strict = true
 markers = [
     "network: tests that need network",