Require tornado 5.1 where selected_subprotocol was introduced

Author: consideRatio

.github/workflows/test.yaml

@@ -95,7 +95,7 @@ jobs:
             pip-install-constraints: >-
               jupyter-server==1.0
               simpervisor==1.0
-              tornado==5.0
+              tornado==5.1
               traitlets==4.2.1
 
     steps:

jupyter_server_proxy/handlers.py

@@ -546,10 +546,6 @@ def select_subprotocol(self, subprotocols):
         https://github.com/tornadoweb/tornado/blob/v6.4.0/tornado/websocket.py#L337-L360.
         """
         if subprotocols:
-            # Tornado 5.0 doesn't pass an empty list, but a list with a an empty
-            # string element.
-            if subprotocols[0] == "":
-                return None
             self.log.debug(
                 f"Client sent subprotocols: {subprotocols}, selecting the first"
             )

pyproject.toml

@@ -50,7 +50,7 @@ dependencies = [
     "importlib_metadata >=4.8.3 ; python_version<\"3.10\"",
     "jupyter-server >=1.0",
     "simpervisor >=1.0",
-    "tornado >=5.0",
+    "tornado >=5.1",
     "traitlets >= 4.2.1",
 ]