test: keep tests off the network

- conftest refuses socket connections outside tests marked integration
- the one test that probed Yahoo for real now stubs the reachability check

tests/test_suite_isolation.py

@@ -0,0 +1,17 @@
+"""The suite runs the same on any machine: no test reaches the network."""
+
+import socket
+
+import pytest
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize("connect", [
+    lambda: socket.create_connection(("192.0.2.1", 80), timeout=1),
+    lambda: socket.socket().connect_ex(("192.0.2.1", 80)),
+], ids=["connect", "connect_ex"])
+def test_a_test_cannot_reach_the_network(connect):
+    """A test that silently depends on a live vendor passes or fails with the
+    machine it runs on; conftest refuses the connection instead."""
+    with pytest.raises(OSError, match="reach the network"):
+        connect()