pylint: disable signature-differs

In Python 3.9, a new keyword argument was added to shutdown().
We will run into this warning everywhere due to that.
It's not yet clear what to do about it, since we can't easily
match the signature both prior to and after that python version.
Let's disable this for now to unblock CI.

.pylintrc

@@ -144,6 +144,10 @@ disable=
     bad-continuation,
     # Nothing wrong with a small number of public methods
     too-few-public-methods,
+    # Python 3.9 is changing the signature of Executor.shutdown().
+    # Not sure how to deal with it yet, but it likely will trigger
+    # this warning.
+    signature-differs,
 
 
 # Enable the message, report, category or checker with the given id(s). You can