Mark code for removal.

Refs #803.

src/websockets/asyncio_client.py

@@ -101,6 +101,8 @@ async def read_http_response(self) -> Tuple[int, Headers]:
         """
         try:
             status_code, reason, headers = await read_response(self.reader)
+        # Remove this branch when dropping support for Python < 3.8
+        # because CancelledError no longer inherits Exception.
         except asyncio.CancelledError:  # pragma: no cover
             raise
         except Exception as exc:

src/websockets/asyncio_server.py

@@ -135,6 +135,8 @@ async def handler(self) -> None:
                     available_subprotocols=self.available_subprotocols,
                     extra_headers=self.extra_headers,
                 )
+            # Remove this branch when dropping support for Python < 3.8
+            # because CancelledError no longer inherits Exception.
             except asyncio.CancelledError:  # pragma: no cover
                 raise
             except ConnectionError:

src/websockets/protocol.py

@@ -1169,6 +1169,8 @@ async def keepalive_ping(self) -> None:
                         self.fail_connection(1011)
                         break
 
+        # Remove this branch when dropping support for Python < 3.8
+        # because CancelledError no longer inherits Exception.
         except asyncio.CancelledError:
             raise