Fix syntax warning about assert

SyntaxWarning: assertion is always true, perhaps remove parentheses?

assert is a keyword, not a function, and using it with parentheses
ends up being interpreted as a tuple argument (truthy) rather than a
boolean assertion and a message

korman/exporter/convert.py

@@ -538,10 +538,9 @@ class Exporter:
                     # my_object.foo = bar
                     # ```
                     pre_result = proc(self, bo)
-                    assert (
+                    assert \
-                        inspect.isgenerator(pre_result) or pre_result is None,
+                        inspect.isgenerator(pre_result) or pre_result is None, \
                         "pre_export() should return a generator or None"
-                    )
                     try:
                         gen_result = None
                         while pre_result is not None: