🐛 fix single_session bug

tests/test_single_session.py

@@ -0,0 +1,28 @@
+from contextlib import asynccontextmanager
+
+import pytest
+
+from utils import make_fake_event
+
+
+@pytest.mark.asyncio
+async def test_matcher_mutex():
+    from nonebot.plugins.single_session import matcher_mutex
+
+    am = asynccontextmanager(matcher_mutex)
+    event = make_fake_event()()
+    event_1 = make_fake_event()()
+    event_2 = make_fake_event(_session_id="test1")()
+
+    async with am(event) as ctx:
+        assert ctx == False
+
+    async with am(event) as ctx:
+        async with am(event_1) as ctx_1:
+            assert ctx == False
+            assert ctx_1 == True
+
+    async with am(event) as ctx:
+        async with am(event_2) as ctx_2:
+            assert ctx == False
+            assert ctx_2 == False