Add parentheses to the line where conn_spec == one_to_one is checked

Previously, one_to_one_cspec was a boolean
if isinstance(conn_spec, dict),
and a string otherwise.
This was obviously a mistake.
Note that everywhere further in _process_input_nodes()
one_to_one_cspec is always used within an if clause,
thus expecting it to be boolean.

pynest/nest/lib/hl_api_connection_helpers.py

@@ -291,7 +291,10 @@ def _process_input_nodes(pre, post, conn_spec):
 
     # check for 'one_to_one' conn_spec
     one_to_one_cspec = (
-        conn_spec if not isinstance(conn_spec, dict) else conn_spec.get("rule", "all_to_all") == "one_to_one"
+        (
+            conn_spec if not isinstance(conn_spec, dict)
+            else conn_spec.get("rule", "all_to_all")
+        ) == "one_to_one"
     )
 
     # check and convert input types