Prevent ResourceWarning about listening socket

Server threads take some times to shutdown, using the server as a
context manager force a call to ThreadMixin.server_close() which
waits on the threads.

Author: bdauvergne

pytest_sftpserver/plugin.py

@@ -8,10 +8,10 @@
 
 @pytest.yield_fixture(scope="session")
 def sftpserver(request):
-    server = SFTPServer()
-    server.start()
+    with SFTPServer() as server:
+        server.start()
 
-    yield server
+        yield server
 
-    if server.is_alive():
-        server.shutdown()
+        if server.is_alive():
+            server.shutdown()