Cancel nursery before closing connection (fixes #5)

This regression was introduced at a8cbe89 because as I refactored I
mistakenly removed the nursery.cancel_scope.cancel(), which is still
needed. This isn't the cleanest way to handle connection closed, but
it's the smallest change that fixes the build, and its based on code
that existed in the last successful build.

Author: mehaase

trio_cdp/__init__.py

@@ -305,6 +305,7 @@ async def open_cdp_connection(url) -> typing.AsyncIterator[CdpConnection]:
             async with cdp_conn:
                 nursery.start_soon(cdp_conn._reader_task)
                 yield cdp_conn
+                nursery.cancel_scope.cancel()
 
 
 async def connect_cdp(nursery, url) -> CdpConnection: