Add integration test coverage for keyring authentication (#3067)

Closes https://github.com/astral-sh/uv/issues/2464

scripts/packages/keyring_test_plugin/keyrings/test_keyring.py

@@ -0,0 +1,23 @@
+import json
+import os
+import sys
+
+from keyring import backend
+
+
+class KeyringTest(backend.KeyringBackend):
+    priority = 9
+
+    def get_password(self, service, username):
+        print(f"Request for {username}@{service}", file=sys.stderr)
+        credentials = json.loads(os.environ.get("KEYRING_TEST_CREDENTIALS") or {})
+        return credentials.get(service, {}).get(username)
+
+    def set_password(self, service, username, password):
+        raise NotImplementedError()
+
+    def delete_password(self, service, username):
+        raise NotImplementedError()
+
+    def get_credential(self, service, username):
+        raise NotImplementedError()

scripts/packages/keyring_test_plugin/pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["flit_core >=3.2,<4"]
+build-backend = "flit_core.buildapi"
+
+[project]
+name = "keyring-test-plugin"
+description = "A keyring plugin for testing."
+requires-python = ">=3.7"
+version = "0.1.0"
+keywords = []
+authors = [
+  { name = "Astral Software Inc.", email = "hey@astral.sh" },
+]
+dependencies = [
+  "keyring"
+]
+
+[tool.flit.module]
+name = "keyrings"
+
+[project.entry-points."keyring.backends"]
+AstralTest = "keyrings.test_keyring"